Hello,
First about the .vst3 bundle folders: There were some changes in the plugin manager in FL 20.5.1 to be able to find plugins inside these folders, but it’s been a while since it was tested so I don’t remember how well it works now. Worth trying if you don’t have that version yet. You could also try the FL 20.6 beta (see our Looptalk forum).
We intend to take another look at this in the future, so we can support them properly if necessary.
About the GUI, I don’t think I’ve ever tested gui-less VST3 plugins as I don’t think there are actually any (released ones) out there. I could be wrong of course.
That said, FL’s generic GUI adds a knob for every parameter published by the plugin, so in theory it should already work.