You’re right, I probably don’t need ECC. I’m mostly worried about bit flips in my important data, and as you say, a checksumming FS and RAID will protect against this while the data is in storage. However, it doesn’t protect against bit flips while copying data, for example copying data to backups - but there are other solutions for this, which I should consider.

Hot swap is nice to have. I haven’t even considered that it wouldn’t be supported by a mobo, I should look into that, thanks. These are the mobos I’m considering for each option:

N100: Topton N100 motherboard, 4x2.5G, 6xSATA, PCIE x1 a.aliexpress.com/_EvVv0k6

8500G: ASRock B650I Lightning WiFi (Gigabyte A620I AX might be an option, but it has only one M.2 slot so the upgradeability is less)

13100: ASRock Z790M-ITX Wifi

The N100 option is cheaper and should be lower power, but as you say I worry about needing another upgrade in a year or so, and this option doesn’t offer much upgradeability so that would mean at least a new mobo and cpu. The other options could accommodate a beefier CPU if needed.