I think you will have to increase your budget, or start with either Macro or Wide and add to it later. Or go 2nd hand...
You're right, with the Sony's. I think they produce some of the nicest / crispest videos underwater given the right lens, dome combination. (Top side they are excellent too)
But as you allude too, they don't seem to handle ambient WB (at depth) as well as Canon or Panasonic, reading the criticism and reviews posted here and elsewhere. It was my No1 reason not to go with Sony's when I went down this route. To be fair Panasonic have only just started being able to produce nice MWB below 20m, so its hasn't been just a Sony problem. The main issue I think is the error, and inability to set a MWB more than just turning the water purple, so people are just running it (as above) in Cloudy WB or Shade WB and then fixing in post.
Give the Sony's some artificial lights and they are spectacular. But I, myself love doing ambient WA stuff so gone with Panasonic, GH3, GH4 and now the GH5 and GH5s
As you are on a budget, and are wanting changeable lenses, you will be looking at the A6xxx or GX series. The A7 or GH line, the housings (and bodies) are far more expensive for the gains in quality against their smaller brothers, but on the other hand with the cheaper/smaller housings, you loose, the mass of the camera, the handles, so that can affect image quality too, with stability, inertia etc... And you might end up buying additional handles, trays and end up at the same size / price.
If I had to choose today, for a trip tomorrow, for $3000, probably the A6500 (if its within budget) if I'm honest, its cheap, the video quality is up there, and would work around any MWB issues i had.
The new GX9 (Dual IS, no low pass filter, better ISO etc) is out - so that could be a contender too if you can wait.
I have been investigating which mirrorless camera is the best value for money underwater 4k video. I film mostly macro/super macro and general wide reef shots.
I looked at the Sony A6300/A6500 (with the Fantasea housing) however the lack of the ability to manually white balance easily underwater (due to the 9900K white balance restriction) gives a lot of the footage an overly blue/purple tone.
Have any users of the A6300/A6500 encountered this issue? Is there any obvious work around (besides dive lights which struggle with penetration).
I next looked at the Panasonic GX85 (with the Ikelite housing). This camera has slightly softer 4k than the Sony and a smaller sensor but is much better at manual white balancing. The image quality seems almost identical to the GH5 which has been lauded as one of the best underwater cameras. Does anyone have an opinion of this camera?
Is there any other suggestions of cameras that I have not investigated?
My initial budget is $3000 for a setup including camera + housing + ports + macro.
I am attempting to avoid a situation where I buy a setup that limits my ability to upgrade the quality. Purchasing a compact and wet lenses now would require a camera/housing upgrade to get to the quality of a dedicated macro lens setup.
I watched your videos on your website. I'm by no means attempting to critique your videos, they have beautiful composition and editing but their sharpness is not even close to acceptable on a 4k screen. Every single one of your videos would've been vastly upgraded if they were filmed in 4k. This is the reason why I am attempting to purchase a 4k setup now, to avoid a situation where a technical limitation of the camera is preventing the maximum potential of the setup from being achieved.
Please do not take this as criticizing your videos, they truly are amazing and have deserved their place at any film festival you showed them.