Switching to Brave browser the KeePassXC integration was the last thing not working for me.

MacOS Big Sur, Brave Browser

There seems to be a bug in Brave that requires you to choose "Google Chrome" in KeePassXC for browser integration.

That made it work for me :)

https://github.com/keepassxreboot/keepassxc-browser/issues/541#issuecomment-541357698