F-droid’s older version is really old. I did try with Aurora, where you can specify the version to download, but I got “invalid APK”… I ended up using a different app (Files) to access Nextcloud’s downloaded files. I will probably end up using Syncthing for more reliability on important files (like train tickets).
Thanks for the help.
P.S. Just saw this message warning about downgrading this specific app (Nextcloud): https://github.com/nextcloud/android/issues/12775#issuecomment-2030852767
Advice: do not try to downgrade by removing the App and installing from F-Droid! All your sync data in the Android device will gone locally and you will need to re-sync everything.