Hi @btrue :
I think your list of changes will work fine. We modified a ROV to dive to the bottom of Lake Tahoe, which is a little bit deeper than Lake Chelan. In this forum post, I talked a bit about coating floats, and linked to a YouTube video that showed the modifications we made.
One change we made that is not on your list is to go to a thicker back plate. I see that Blue currently rates their 14-hole 4" endcap to 500m, so maybe its not an issue. We were worried about it bowing at depth, so we custom-machined a similar 14-hole endcap out of 10mm aluminum stock. The 18-hole endcap is already made from this thicker stock, but Blue had not released this at the time we were doing our modifications.
To amplify what @nperry said, a fiber tether is not a requirement for doing basic work at 500m. You can use a VDSL modem for this purpose, and I’m guessing that even the RAK V30 board would work fine if you want to stay with homeplug. That being said, if you feel that you’re going to graduate to multiple cameras, or very high definition cameras, then you will probably need fiber at some point. Fiber tethers can be pretty fussy, and have handling restrictions related to minimum bend radius. For most general-purpose diving we don’t consider them to be worth the bother.
Walt Holm
Mission Robotics