Your only choice would be a mirror, as you only have two bays. RAIDZ2 needs at least 4 drives to be useful.
Have you checked that you can actually run a different OS on the UGREEN NAS?
Considering the services you want to run on the machine you’ll want all the RAM you can get.
Also, the CPU is already weak by today’s standard, especially with “heavier” workloads like Immich machine learning.
You didn’t put a country, but wherever you are: check the laws regarding underage relationships, if any. He might actually get into trouble depending on what they’re up to.
XMPP? Matrix?
That’s cute, but very much a bubble view. Usually not worth the effort unless the devs themselves are users.
Why do I need a messaging system in the first place?
You might not need one, but the majority of users want and/or need one for user management, password reset, notifications etc.
And it is being developed for the majority of users.
Do you know of any other near-universal messaging system to use instead?
Edit: also, the downsides you mentioned depend really hard on the email service you choose to use, or choose to host yourself.
self-hosting
and people who want it to work with minimal friction.
We’re talking (potentially) highly sensitive contents here for the most part. Yes, selfhosting has become easier than ever, but at the same time more people who basically lack the experience and/or patience to actually understand what they’re doing want to start selfhosting. And that simply doesn’t end well in an alarmingly high number of cases.
Yeah sure, of course there are tools that can make life easier. But have a look at the “big” self-hosting packages. A lot of them will need at least some manual configuration. Then there’s the “exposing a host to the open net” aspect, which can (and usually will) introduce a whole different level of attack surfaces.
So combine that with the ever-growing number of self-hosters, and of course you will notice more advice like that.
Nothing besides a mirror makes sense with only 2 storage bays. The overhead would only increase for absolutely no gain.