Then there is QNAP. Apparently their system QTS is not as polished as DSM, but everything needed should still be there. There’s similarly priced, similarly equipped TS-462. It’s just dual-core CPU, but has more RAM (not upgradeable though) and it seems it can accept M.2 as storage at least. As per internet research, the build quality is just as good as Synology.

I have the QNAP TS-462. It’s my first and only NAS so I don’t have any points of comparison for you. Feel free to ask any questions you have and I can try and answer for you.

I do want to point out that the RAM is upgradable, but, unintuitively, only on the smaller 4GB RAM SKU; the 8GB RAM SKU has the memory soldered on.

I learned this the hard way and had to return the first one that arrived, but I currently have the memory maxed out at 16GB.