samba. share a folder on pc, and on your phone use a file manager that can access smb folders in your local network, then just copy or move from or to that folder. bit of a hassle to set up the first time, but makes things more convenient in the long run.
try disabling or enabling hardware acceleration. i think its in the vivaldi:flags page. also did you install flatpak version? you could try and download the .deb from their website and install that to see if they are any different. i was a vivaldi user too but ended up ditching it because of some bugs in the linux version.