The failure to sync randomly about 30% of the time with the UWT trigger is a real problem.Sad to hear that the UWT trigger does not work with your Sony. It is hard to believe that this is a real problem...
The Nauticam trigger, which is manual only, will not install in the A6400 housing, I have not tried the SeaTurtle trigger and if I did go that way that would require the one they call the Moby due to space limitations. I have good sync using the onboard flash. The real solution is to do what I should have, go with a different camera like the OM that has a cancel function for the pre-flash. I think that is the root of the problem with the UWT trigger, it fires on preflash sometimes. Again, regardless of settings, regardless of either of my two cameras, regardless of various strobes, regardless of WL command or not, regardless of trigger switch selection including the manual switch position.
Sony RX series, A6XXX series and some of the A7 series (maybe all?) always fire a preflash regardless of the camera being set to manual as there is not a manual strobe menu selection with selectable power as their is with Canon, Olympus, Nikon and everybody else .
If there were a simple manual trigger with preflash cancel like the Nauticam trigger that would fit inside the Nauticam NA6400 housing that would be great, but then you cannot shoot TTL as an option and even if that was acceptable the Nauticam trigger will not fit inside the housing. Now it will for those cameras Nauticam specifies and makes provision for their trigger. And I do believe the Nauticam manual only trigger works just fine but again sacrifices the choice of TTL. This isa case of not being able to have your cake and eat it too.