KeepassXC has built in db file merge support! It's hidden somewhere in the menus. I've never personally had to do something more complicated than "I added a new entry on two different devices before syncing" but it's worked well for me so far.
I saved my database's password as an entry just to quickly merge it with the .sync-conflict version. It takes 2 seconds, and I don't worry about losing anything anymore. Works beautifully for me. KeePass could check for such conflicts (syncthing, dropbox or else) in the directory where the open database is stored and suggest to auto-merge.
