I’ve never tried having the app on multiple devices - I specifically didn’t want it on my phone - but it’s worth a try. I use whatsapp web in firefox, and only start waydroid when I need to log in again. The third party linux apps just load whatsapp web in electron or something.
p2p from behind a CGNAT works just fine as long as a single server is accessible and can mediate connections between other peers. Most non-servers are behind some sort of NAT these days.