Iirc, the appimage doesn’t have the sandbox issues from flatpak. I’ve never had problems using the appimage version (but under Opensuse). Then again, SQlite is sufficient for my needs, so that removes one external dependency.
Otherwise, you could try the OBS, if you know what version of Ubuntu your distribution follows (but no guarantees), or compile your own version. I’d try the appimage first (with prior backup of the databases, the database layout has changed between versions 7.x and 8.x)
Well, I’d have thought the normal Ubuntu repository would be preferred. There, they are up to version 8.0.0 afaik.
And, you still need to know which version of Ubuntu your Elementary OS is based on…