Well I’m not sure what did it (I suspect installing a GNOME application which fixed some XWindows configuration) but I got Opera working, as well as some other graphical based software that wasn’t working properly. I’m not sure what I did but I did keep trying fixing things until I got somewhere.
These helped:
sudo apt-get update && sudo apt-get dist-upgrade
To get snap working (something with a problem with the file system) I had to configure, build and install some software, as per here:
https://www.reddit.com/r/Crostini/comments/apvcj8/snap_working_with_added_fuse_support_in_m73/