Thank for the suggestions, folks. Some comments/answers:
1). FF - I agree with @Chris Ross. I have been shooting wildlife, nature, etc for 20+ year with everything from an 1D2 (all 8MP of it) to an 5D4 to crop-sensor X/XX-D bodies, and i dont really see “full frame” as any kind of holy grail or APS-C as something one has “settle for” as a lesser alternative. I get that people feel differently and that’s fine - APS-C is more than good enough IQ for real world use (prints, online) and the appropriate FOV, i prefer to get the smallest/easiest-to-travel kit i can get away with. Let me put it another way - no one ever looked at a well-composed, properly-processed image and went “oh, this is nice but it would have been even better if shot on FF” and back when i used to sell my images, no one cared what
Underwater, I actually switched from a Canon DSLR to MFT as the latter met my needs better, but they seem to have plateaued as a platform and are being left behind by the newest sensors from Canon/Sony, which is what is driving the change (first i need to migrate my wildlife stuff over to newer RF lenses though for their much better IS). But i am no tearing rush, as my current rig is still doing the job.
2). As you can guess from the above, i dont have Gear Acquisition Syndrome when it comes to camera stuff. I dont pixel peep at 100% and if i can find a less expensive alternative that meets my needs, then i am perfectly happy going that route. In this particular case, i that intermedia WA range isnt something i use too often: my first WA lens was a Sigma 10-20 but once i got my Tokina 10-17FE, i never once put it back on the camera again.
Plus, if i do like it, i will likely think of a way of incorporating a more long-term solution into the next housing, whenever i get around to upgrading. It likely will not be a N85-based Nauticam system, which will render this lens pretty much useless.
So i dont want to spend that sum if there are less expensive wet lens alternatives available - hence my question. If there is an option that gives me pretty good results, i will go with that. If this is the only alternative, i will likely not bother - as @JustinO said, that makes more sense than spending $2.5k for a lens that i will only use for a year or so.
3). As it turns out, i actually have a Tokina 10-17 FE left over from my Canon housing days - that was my favorite lens and is one of the reasons why i am leaning towards going back to Canon.
4). Yeah, a 14-42 plus port plus WWL is another option,, as is the 10-17 with the adapter and other port suggested by Chris - i had in fact considered the former a year or so ago. But that means traveling with 3 ports, and giving up on macro opportunities, and again, generally, the idea of adding more adapters, ports, domes, etc just messes with my photography zen. I find myself moving to a more minimalist approach to all my photography - for travel/street, i have gone from going with 3-4 L lenses and 2 DSLRs to a Fuji body, 2 primes and a small zoom. So the idea of a 50-60mm (in FF terms) lens and a wet adapter for WA really appeals to me from a simplicity of travel and setup POV.
5). @Barmaglot yes, i did find the Weefine and Kraken lenses. Would you, or anyone here, have any experience with their IQ? Given that the sharkies are likely atleast 2-3m away (else i will be regretting not attaching my FE!), the water column will likely be the limiting factor in resolution anyway.
Thank you everyone for the suggestions. My apologies for seeming to push back on some of them - it isnt so much that, but at this point, i have fairly well-developed preferences on how i like to shoot and with what, and so am trying to see if there are options that fit that. If there arent any, then i will certainly have to check alternatives. So i am making a note of all the suggestions here and i do appreciate people taking the time to post them..
Some of my photos, in case anyone is interested:
https://vanditkalia.com/?page_id=13
(And other stuff one level up - the website is a perennial WIP. I like taking photos. Setting up/updating websites, not so much)