Does just getting an intel NUQ not suffice? i bought one for 180 and it works great. runnint ~16 apps + an MC server and no issues

That's not really comparable hardware. The AS6804T has four 3.5" HDD bays, Synology hardware generally offers two to eight bays. They're NASes, the NUC is just a mini computer with little to no built-in storage capacity. It's not about the apps here.

I already have an infrastructure like that. Mine is running Debian Stable, a couple of containers for background jobs and a couple of daemons.

However, I need to backup a lot of things, and ensure that they don't bitrot. A decade old photography archive, meticulously ripped CD libraries, a full cloud storage backup, etc. etc. Plus I don't want to dig disks to get a single file which I don't want to put on somebody else's computer (i.e. cloud storage).

This needs a two tiered solution. Flash based hot-data area for the running daemons and a spinning array for backups. Both RAID (to be able to scrub and repair bitrot).

The problem is, I'm a sysadmin. I see & use big storage systems and know what they are capable of. I want the personally useful subset of this at home. Plus I want to make it accessible to other people at home, so their files will be safe, too.

This means at least TrueNAS and 4-6 disks to begin with.