Nice!
Regarding the Accessory's, I've just opened up the GCS and give it a go and see how to get it working so what I did was...
Bare in mind that I've never used Accessory's before, didn't need it and by all means this doesn't have to be the best way to do it but it worked!
Go to Configuration - Inputs - RC INPUT, now enable Accessory's channels by selecting their type, in my case it is PPM.
Then assign a channel number to that Accessory, in my case Channel 6.
Again, this is how it is In my case, like shown in the image.
Now re-run the Transmitter setup wizard and that should be it.
Important notice, do set all switch positions to their neutral when asked, so your 3 position switch's (aside flight mode switch) have proper curve/value assignments
If this doesn't work, then you probably haven't assign properly a channel to a switch in your Taranis. Please let me know if that's the case so we can see if there's something turned on accidently or something else.
Hope it helps
Best,
Dragomir