The main problem with all the alternatives is for me (as a hobby photographer) the lack of models on these platforms. When looking for models, I find them on Instagram and no other platform. As with WhatsApp the majority of "normal" people have decided to use that, so if I'm telling them to contact me on Signal, they shy away from that (and stillI I refuse to use it as much as possible).
So looking at Signal, it's free and very, very close to WhatsApp and yet still people don't want to use that. Getting them to use pixel fed would be much, much harder.
After driving my current car with Android Automotive for over 45.000 km I can only advice to stay away from vehicles using it
Android Auto (with the car working as a display for your phone) is really nice. AA OS is not.