I’m not super experienced, but I just got file sharing working on a couple of Mint machines.
I had to install Samba and create a Samba user to be able to access from another computer. (I just made the Samba user the same as the computers’ login)
It was a super simple terminal line to do so, but I’m at work right now and don’t have access to the tutorial I bookmarked.
I also followed a tutorial to add right-click shortcuts for sharing in Thunarr (not sure what you’re using for a file browser)
Oh, but when I do connect to one machine from the other a dialogue asks me if I want to forget the password immediately, remember until I log out, or remember forever. Maybe that’s the piece you’re missing…