Actually, the next time, if you run any linux distribution on your pc, then I would recommend to install imapsync there and do the one-time syncing, this avoids having to install additional packages alongside of Cloudron on that server.

The imapsync tool will work as an external client, just like any mail application like thunderbird. Basically the details about ports and connection type from https://cloudron.io/documentation/email/#accessing-cloudron-email have to used. Note that apps like sogo, which run on the localhost network on the server itself, use different ports.

This is currently not supported. Port 22 is one of the hardcoded reserved ones. I am not sure if it may work, but if you do not have sshd running on that port, then you may be able to create a custom iptables rule, which does the portmapping from 22 to the one used by gitlab. Maybe https://www.cyberciti.biz/faq/linux-port-redirection-with-iptables/ helps here, especially:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 22 -j REDIRECT --to-port 8080

We still have to wait until https://github.com/nextcloud/server/issues/11670 is resolved unfortunately.

Cloudron will "take over" the server and also manages the packages and services, which are installed. Generally it is not supported to install anything on the side. In this case Cloudron depends on mysql-server-5.7 which will probably uninstall mariadb through apt.

There is currently no update on this. As you mentioned earlier, if you can mount an external storage as a local filesystem, then the filesystem backup backend may work. The filesystem should support hardlinks though, you can read more about that at https://cloudron.io/documentation/backups/#filesystem

There is a work-in-progress guide how to use it against gmail at https://cloudron.io/documentation/guides/mail/ since it requires a few special attributes, maybe it is useful for you as well.

So far we had a good experience with using the imapsync tool to sync full inboxes for users.

@RoboMod sorry for this, I have reactivated it again and saw that I previously had suspended it, since your server didn't respond to our alive check for some time and I thought it was unused.

The proposal from @msbt is currently the only supported way, as Cloudron does not support cross domain aliases. Also rainloop is setup in a way that it only works with one domain, the one where it is installed at. To create the server side sieve rules for forwarding, you could also use other mail clients instead of Rainloop, which support sieve. One example is Thunderbird with the sieve plugin.