Hi Hans,
Which image did your phone come with or did you try? For me personally, Plasma seems very unstable at the moment as well. I had a ‘Braveheart’ edition (2GB), but upgraded the motherboard and now have a 3GB version like you. Since then Plasma is nothing but trouble for me.
However, all other images (such as Mobian, postmarketOS (which you should build yourself through pmbootstrap), or Manjaro’s own Phosh for Pinephone) seem to work just fine! Did you try any of those?
Hope this helps 
Best wishes, Thomas
PS: Switching to another image/mobile DE won’t solve all the issues you described, but hopefully at least bring more stability.