So this is the end of it. There were no more difficulties than the three stated in the first message:
- emails
- incoherent nginx configuration files for domains causing errors.
- app restoration impossible at first because of existing permissions
The fixes which worked for me:
-Regarding emails, I did manual copies from the backup tarball to the new system. - Regarding the nginx confs, I deleted and re-added all my domains manually.
- Regarding app restoration. I followed a cycle
yunohost app install <myapp>=>permissions already exists error=>yunohost app remove <myapp>=>yunohost backup restore <mybackup> --apps <myapp>. It worked like this for all the apps but vaultwarden which needed after “sucess” in restoration a forced upgrade.
I also had achownerror during my first attempt of nextcloud restoration but this was an old problem due to uid < 1000 and unrelated to the restoration. Fix is in One of my Nextcloud installs is missing after a system upgrade - #3 by colmoneill .
Finally, this migration went fine with a bit of work, thanks for your help @gauthier67 and @Aleks !
Best regards,