I thought I'd be able to migrate between versions 9 and 10 fairly easily.
You could (though many of the issues are down to Redhat, not us).
But there are still lots of dufferences, just as there were wuth previous versions.
Use the correct methods and it isn't hard. Remember this is SME, not just any old system.
I'm going to run my tests directly on version 11.
You can but be aware it is Beta so may have issues (see bugzilla), many contribs are not yet migrated, and it may be even harder to jump from 9-11
I was able to install using the netinstall version.
https://wiki.koozali.org/Rocky2smeYou really should spend a lot of time reading the wiki.
Backup/Restore
The only way to really do this is backup/restore from a v10 backup. Anything else may cause issues.
You could have used this for v9-v10 and should be able to for v10-v11
https://wiki.koozali.org/MigratehelperAgain, read the wiki. There is a French version too...
Your RSA key issues will be to do with the fact that security has increased massively since v9 and your old keys are deprecated.
Hence use the correct methods to upgrade and save yourself a lot of trouble.
I'll have a big job left: modifying my ibays with my pages from PHP 5.3 to 8.4.
If you had kept up to date properly as advised you would not be in this position.