I've been looking into a NAS myself.

I think self-built is the best bang for buck you're going to get and not have any annoying limitations.

There's plenty of motherboards with integrated CPUs (N100 same as cheaper Ugreen ones) for roughly 100. Buy a decent PSU and get an affordable case. For my configuration with a separate AMD CPU I'm looking at right around 400 Euros but I get total control.

And as far as software is concerned, setting up a modern OS like TrueNAS I find about the same difficulty as an integrated one from Ugreen.