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.
For a NAS, I don't think I'd need more than 1-2 lanes for any single device. That sounds fine.
Shoutout to openmediavault. Just yesterday I installed it on my DXP8800 and now it works like a charm. But to install another OS you have to deactivate the watchdog timer in the BIOS, otherwise it resets the NAS every three minutes. Press CRTL + F12 to get into the BIOS and look for something like "watchdog" and disable it.
Last but not least, they seem to have Docker support which was restricted to more powerful Synology models, and it's a nice bonus for self-hosting nowadays.
Ended up buying a terrmaster DAS instead and connected it with usb to my NUC.
Also considered a NAS enclosure with an n110 mini itx board, would allow you to upgrade it in the future.