It’s not too complicated but you don’t get some things for free like with Synology. It require work to setup scripts for offsite backup for example whereas Synology has a backup app with a UI.

For storage, I used to run ZFS in a raidZ2 configuration. If you do this then I suggest having a cron job running a script that can alert you if the pool is unhealthy. This is again something that Synology does for free.

You could also look up trueNAS core and see if that’s something that fits for you.

I own a Synology NAS. It’ll be the first and last one I buy. When I need an upgrade I’ll go back to building my own again.