I've been using Zimbra's free edition for quite some time myself too, since v4 IIRC (back before it moved from TomCat to Jetty). Ran DayJob's mail server from it for a fair few years too.

> The free version has no backups which is a pain and …

I “simply” rsync the key parts off to my usual backup locations (where snapshots are made etc.)⁰. There are a few articles on how to do this floating around. The restore procedure is basically the same as transplanting a Zimbra instance from one install to another¹. For paranoia's sake I have a small extra VM that automatically restores itself from the latest backup daily, and I check occasionally to make sure it is running fine and has recent mail. That VM is not publicly addressable, but if my main instance dies irreparably I could make it so easily and flip DNS over to point its way, probably bumping up its RAM allocation too, losing at most what happened since the last backup. The VM actually thinks it is the “real” primary instance, even with the same local network addresses, as they live behind a split DNS arrangement. A copy of this VM can be used to test upgrades and other major admin to an extent too.

> makes me scared to reinstall...

I've found upgrades work smoothly, if you wait a bit after major versions to let the quick adopters iron out the issues first, so I've not had to completely fresh install. Just don't upgrade the base OS at the same time as Zimbra. For OS upgrade when an Ubuntu TLS release nears its own EOL or Zimbra's support for it does. I do by reinstall rather than upgrade but for Zimbra in the process things are the same as the backup restore operation. There has always been sufficient overlap between Ubuntu LTS lifetimes and what versions Zimbra supports that this has worked for me.

> makes me scared to … move away from

I've successfully used imapsync³ to move mail between various servers in the past, including into, out of, and between, Zimbra instances. Obviously that doesn't cover calendar data, config, and other features, if you use them, but for mail it is a good option. Regular syncing to something else could be an extra fall-back backup option too though I've only used imapsync for one-off bulk transfers.

> Zimbra 9 looks nice but licensing.

Same. Given how many times Zimbra has changed hands I'm surprised similar hasn't already happened sooner and I've had a good run with it! Maybe closer to 8's EOL others will start maintaining reliable builds of the OS parts, though I'm thinking I'll shift to something else next year. I've mainly stuck with Zimbra because it just works, given my needs have changed considerably over the years something lighter will probably be better and if I'm going to have to do more work (or pay, of course) to keep using Zimbra it is definitely time to properly reconsider the state of the many alternatives. At least we've got a decent amount of time to transition.⁴

[0] Hot rsync to local LAN backup first to speed up the part where the instance needs to be down, stop Zimbra, rsync to local LAN again to get a consistent backup there, restart, then soon after that the local backup is rsynced off to the offsites.

[1] Install same OS+Zimrba versions, stop, restore data from backup over the top, some steps to check/fix perms, off you go. Again, there are examples in the docs & forums.

[2] As [1] just with the new OS.

[3] https://github.com/imapsync/imapsync

[4] IIRC 8.x is due to EOL and stop getting security updates at the end of 2023, I'm intending to have something else in place by the end of 2022. If I have time to look around properly and make a decision by then, maybe I'll get around to it over the extended bank holiday weekend at the start of June.