Barmaglot

Take a luggage scale, hook your assembled rig up to it, and submerge it in water (the rig, not the scale). The resulting figure will be the amount of buoyancy that you need to add. In saltwater it will be a little bit more buoyant than in fresh water, but the difference is negligible unless you take it somewhere like the Dead Sea.

If it's just under the surface, have you checked whether or not wifi from a smartphone will reach?

Oh yes, now I see it. I suppose, however, that this is something that should, in theory, be possible to use with any housing that has a bulkhead for an HDMI cable, unless it relies on power supplied from inside the housing and uses a custom connection to facilitate that.

I don't see anything that looks like a viewfinder on that Facebook page, but I suppose it's technically possible to build an external HDMI monitor with a small, high-density display and a mask-adapted eyepiece, effectively an external EVF instead of a regular external display. I can see a number of advantages to this approach - you could have arbitrary orientation and tilt, for one, instead of the fixed 45-degree or straight optical adapters that are prevalent in the industry, you get an extra stabilization point for your camera, and at the same time avoid the weight, bulk and drag of a typical 5-7" housed monitor and occlude all the potential sun glare. You could also take a page out of Marelux book and integrate some dive computer functionality into the same display. On the downside, there is no way it can be anything but eye-wateringly expensive, and, depending on camera and electronics used, it could suffer from some display lag.

...and just like Easydive, it does not appear to have any knobs besides lens zoom, i.e. shutter speed/aperture/ISO/exposure compensation/etc have to be managed by button presses instead of knob/dial rotation.

So, um, why do you have compass on top of your camera if it's so inconvenient? Why not on your wrist alongside everything else? Also, some of us use screens rather than viewfinders for composition. On DSLRs the viewfinder was important; on compacts or mirrorless it makes little difference.

Yes, obviously you can look left and right to check the computer on your wrist, but having it in front of you on the camera is more convenient.

Convenience - when you're constantly looking at the camera screen and/or viewfinder, you can check your dive computer for depth/time/NDC/air without looking away. Marelux even makes a 'smart viewfinder' with an integrated depth/dive time/temperature/ascent rate display. This is particularly valuable on blackwater dives where you have little in the way of visual reference as to your location and attitude. If you're worried about losing your computer with the camera, then you should also be worried about your dive computer failing mid-dive, so you should also be carrying a backup, and if you're carrying two, why not mount one of them on the camera?

Problem is, that will require another new camera. A new a6400 is $900, so just the housing + body is approaching $2k, and the jump in capability from a6300 is very incremental. It's got somewhat better AF, but not the big boost found in A7RV/A1/a6700, and the rest is basically the same - same 24MP sensor, same small battery and so on. Might as well just get a camera repair shop to replace the flash on a6300, get a new tray and keep using it. I'm actually very unimpressed with Ikelite's a6700 housing. It's got a thumb trigger for AF-ON, yes, but no access to front dial, making three-dial operation impossible, and the knobs that are there are located in such a way that you have to reach for each one individually. There are no M14 or M16 bulkheads, and no provision for optical triggering - electric sync cords only. I suppose a UWTechnics converter can be installed in place of the electric bulkhead, but that's a $650 part on top of a $1200 housing.

While in general I would agree with you, I have met several people using the 16-35mm f/4 Z lens behind a SeaFrogs 150mm dome and the results seemed adequate. IIRC someone posted, either here or on Wetpixel, tests showing this particular lens being more tolerant to smaller domes than most rectilinear ultrawides. Canon 8-15mm is also an option, and I'm pretty sure that a SeaFrogs dome will give it better positioning than, say, Ikelite compact 8-inch - if nothing else, it's a much deeper section of a sphere. If it was something like an A7 III then maybe, but A7R V is a current-gen body with no replacement in sight. A quick search shows none to have been posted in the classifieds here thus far, and only a single offer each on Wetpixel and Scubaboard, both over a year ago and with no price stated. A new Nauticam A7R V housing is about $4500, compared to SeaFrogs $566. A Nauticam setup with a 180mm dome set up for 16-35mm f/4 and a flat port for 90mm, per Backscatter, runs about $8300, whereas a SeaFrogs setup for the same is about $1500, both including a vacuum system and a manual strobe trigger. The $6800 difference buys a lot of air/nitrox fills...

I'd say, the main thing you'd be losing is lens selection flexibility. As far as ergonomics go, it's kind of a wash between the two. Most Nauticam housings have nice thumb triggers for back-button focusing, which allow you to operate the housing while holding on to tray handles, using your thumb to focus and index finger to shoot, but your a6300 doesn't actually have that, so it's not a factor. Likewise, Nauticam offers nice magnifying viewfinders, but it doesn't sound like you have one of those either, so you'd be using the camera screen in both cases. I don't have personal experience with the SeaFrogs A7RV housing, but I do own the A6700 housing which has a similar control layout, and the ergonomics are surprisingly good. It's not as good as a modern Nauticam or Marelux housing, but if I put my right hand on the housing (i.e. between the housing and tray handle), I can have my thumb on the AF-ON button or the top dial (aperture), my pinky on the front dial (ISO), the ball of my thumb on rear dial (shutter speed) and my index and/or middle finger on the shutter release, enabling three-dial operation and back-button focus without moving my hand at all. My left hand is on the respective tray handle and easily reaches the zoom knob. This might not work if you have especially small hands, but mine are fairly average and I have no trouble operating all the controls. As far as lenses go, you'd be using Sony 16-35mm f/4 or Tamron 17-28mm behind a 180mm dome. I believe Canon 8-15mm can be adapted and used with either a 150mm or a 180mm dome, but alignment can be an issue, and you'd need to make your own zoom gear. SeaFrogs don't have extension rings - what's labeled as extension rings on seafrogs.com.hk are actually dome port bases; you can't stack them or use them with flat ports to customize lengths. There is no option for 100mm or 230mm domes either. For macro, it's basically Sony 90mm or nothing. The new Tamron 90mm macro should fit into the same port, but as it is slightly shorter, it won't work as well with close-up wet lenses for supermacro, and f/16 minimum aperture is a weakness. VPS-100 vacuum system is mostly useless - its seals degrade when submerged and after 70-80 dives or so it starts leaking water. Fortunately Vivid Housings makes a version of Leak Sentinel that fits SeaFrogs housings; you can use that. Since A7 series cameras lack a pop-up flash, you will need a way to trigger strobes. Avoid the SeaFrogs optical trigger - it's cheap, and actually surprisingly powerful, as it is a small Xenon flash rather than an LED, so it works well with low-quality fiber optics, but it has a major flaw - it draws power while in standby, and the battery only lasts a few hours, so basically you have to turn it on, seal the housing half an hour before your dive, vacuum it, and it's good for one dive only - you'll have to recharge it before the next dive. A Turtle or UW-Technics trigger is a workable alternative, but much more expensive. Another option is an electronic sync cable, if your strobes support that, but that means messing with a bunch of extra o-rings.

To be honest, I don't recall ever having issues with flaring from dome, but I've been shooting with Retra strobes and either SeaFrogs acrylic domes, or AOI UWL-09F wet lens. I can only recall getting flare from sun on the AOI lens.

Without diffusers, Z-330s produce a pronounced cross-shape light pattern - https://www.retra-uwt.com/pages/flashgun-light-comparison - it's possible that its horizontal spread is more intense than Seacam's more diffuse circular beam. If I remember correctly, Z-330s have a small rotating hood specifically to control this. Also, I'm guessing it wasn't an apples to apples comparison, as you weren't comparing different strobes on the same rig, but rather two completely different rigs. Flare resistance is heavily affected by glass coatings - what domes were the two of you using? Did either of them have a shade?

I suppose any potential housing for it will need a dome port?

Universal housings exist; they're made by Easydive. It does mean some compromises though, for example all functions have to be controlled by buttons, no knob operation.