I believe in KDE you can use KDEConnect on the phone and on the computer. This should allow the phone to appear as a mounted drive in Dolphin, and thumbnails should then appear.
Thanks for this. I don't see that option, but I'm on a Samsung S22, so it may mean that "feature" hasn't rolled to us yet. Just updated Fennec from F-Droid.
I use nicotine+, and you have to set the port you're using, and either make sure it's forwarded, or turn your router's uPNP setting on so that nicotine will handle the port for you.
Your apps going to sleep seems like a common problem. I thought there was a setting in Android to prevent certain apps from being put to sleep. That might help.
Check out slsk.