I've found UW specific fiber cables quite thin and flimsy (and expensive!) so I've taken to making my own out of 3mm end glow cable, mounting it into the plug ends from a failed cable. There was a recent thread about this exact issue with S&S strobes & led triggers with a video someone posted making it work with a custom cable like mine.
Fiber cable is super cheap. Cut, flame polish the end, mount into the plug and you're good to go. Someone else suggested dipping in boiling water & wrapping around a rod to create a coil in the cable. Plan to try that.
I've built my own FO cables as well. Seemed like a no brainer - existing ones I had were so bloody thin and flimsy and cost a fortune. When they inevitably failed, I cut the plug ends off and mounted them to some decent size end glow (or could use multi strand). FO cable is cheap. You can get the plug ends for a couple bucks from some of the UW photo shops.
I understand how TTL makes everything underwater quite a bit easier but I am confident that with some trial and error, my experience shooting almost entirely with artificial light on land will allow me to work within the limitations of my camera setup.
I also work a lot with artificial light topside and I'm confident you'll be totally fine in manual UW. Its really not complicated, at least regarding strobe power, when you have experience working with these tools.
Get the D500 and keep your 10-17. That camera is a beast and the 10-17's versatility and compactness with a small dome is unrivaled underwater. Ignore the "FX are the pro cameras" mantra nonsense. Mostly came from Nikon ill-serving the DX line for a few years.
I've shot a D500 alongside my D810 topside and would seriously prefer it for certain things like macro. Image quality is 100% up to par, though I don't pixel peep.
If you can easily do that Type3 to Type4 conversion, Subal makes the most sense to keep ports and viewfinder. Otherwise if you're in Canada check out Aquatica, I'm on my 3rd housing since 2006 and they've been fantastic. Prices are very reasonable, perhaps more so from within the country.
The other option would be to just switch to a mirrorless system like Sony or Olympus. They're so compact and image quality will still blow you away when paired with quality lenses.
This is ongoing issue in all photo forums above or below water, suddenly you have extra resolution and when you look at the corners at 100% in Photoshop you discover that the image does not look as sharp as the lower resolution older model, therefore the new camera causes poor corners. In fact the corners are the same as they always were and the image will look the same if you downsample to 36 MP or whatever the resolution of the previous model was.
The actual situation is you can't take advantage of that extra resolution in the corners because the lens does not resolve that well in the corners.
Indeed, well articulated, but has it been definitively shown somewhere? Personally I just don't see it both topside and underwater.
All I've read are allusions to the idea on forums - primarily before the D800 came out in 2012. I've not seen much mention of it since (thought it was debunked) until here and now with the D850 UW.
Of course I've always been a firm advocate for using the best optics possible and when asked I advise people of the importance of lenses vs cameras (and of shooting vs fussing about gear).
I can somewhat see the idea from a "get-the-most-from-your-high-res-camera" perspective but not that one's image will suffer due to some larger relative discrepancy between the sharp center and unsharp corners. That acceptable optics on a D810 (or even D700) may no longer be such on a D850.
I shoot a lot and just haven't seen it. Though I'm not much of a pixel peeper.
I can't recall ever hearing in the film days that soft corners were blamed on film having too much resolution power.
I believe that the current best wide angle/dome combo for 36mp camera is also the current best combo for 45mp camera.
A big 'nay' to that one, both topside and UW. Not watched the linked video but I stopped using UV filters years ago. Even good quality filters can be detrimental to the image, seen it myself. Also, in many years of topside shooting, haven't yet felt the front of a lens threatened as to require protection.
Lens hoods however are a different story. When you drop your camera, as I've done multiple times, it will impact on one corner of the body and on the front of the lens. A hood will save the lens. An L-plate likely helps absorb the camera-end of the impact too.
You don't need no stinkin' TTL. Never used it UW, never wanted to.
I've used mixed strobe brands for a couple years now (Inon, Ikelite) since my 2nd Inon flooded. Its not ideal but works fine as long as I gel the Inon to match the Ikelite. I have them very close using a mild CTO, I think 1/4.
Im in the water year round and my 2 primary pairs of fins (full foot cressi garas and open heel mares avanti) are over 10 yrs old and just starting to show wear. I often don't even rinse them. Too busy taking care of my camera and bc/reg. Fins are the one thing you don't need to worry about.