Looking at Nextcloud releases, it seems that they do a major release once a year.

It should be possible to catch up with the upstream even if there is a single update per Fedora release.

Update to Nextcloud 11 in Fedora 27, 12 in current Fedora 28 and then in Fedora 29 cath up with upstream.

Admittedly that will be a lot of work, but it keeps instances upgradeable.