Well, here we go again. I moved a Split4k from an F405-CTR (where start/stop worked perfectly) to an F405-WING. On that FC I used the same UART I’m using in another wing to control a Hybrid (UART 6), which also works perfectly there. F405-CTR was AC, now it’s AP (obviously).
The settings are the proven ones, yet start/stop won’t work:
RCn_OPTION = 78 for start/stop
SERIAL6_PROTOCOL = 26
CAM_RC_BT_DELAY,7000
CAM_RC_BTN_DELAY,300
CAM_RC_CONTROL,0
CAM_RC_FEATURES,247
CAM_RC_MDE_DELAY,800
CAM_RC_TYPE,3
One thing I noticed is that the features populated by querying the cam include all boxes ticked - except “Stop recording”. So I ticked that one manually but even after many attempts the cam won’t react to the switch I set. So what is it this time? 
And something I noticed while working on this and really got me excited: There is now SERIALn_PROTOCOL = 27 - Smart Audio?! I found this page but it looks a little broken on my browsers: https://ardupilot.org/copter/docs//common-vtx.html
I was really waiting for this. So far I’ve been controlling SA VTXs directly from the Crossfire RX but of course it’s much nicer to have it integrated into AP. Also some VTXs (Rush, for instance) don’t understand the SA signal coming from the TBS RX so I had to press physical buttons on the VTX with those. Can’t wait to try it out!