A couple of comments. The sony a6500 doesn't allow manual control of the strobe beyond setting it to like -3 stops, you can't just dial it really far down and use it to trip strobes in manual mode, the internal flash only fires TTL.
Apart from Ikelite's housing no one has TTL working with wired strobes for faster recycle, I tried. Aquatica has a nice housing with a wired bulkhead for Ikelite, but it's not TTL, weirdly their SLR housings all license the tech from Ikelite for TTL but not their mirrorless housings, this is not super obvious from the web site but I got one of the housings and validated this myself, and then returned it.
I ended up selling off the Ike's and going with Sea&Sea's although the Retra's weren't available when I did this, doing it now I'd likely use the Retra's or the new Inons.
The camera works great in the housing, I'm very happy with it, for a complete write up look here: https://www.scubaboa...housing.549228/