I believe the RAM calculation is less important for ZFS these days. I capped mine at 16GB for 64TiB useable pool and had no issues. (This was zfs on linux which i think Truenas Scale is based off anyway).
Regardless unless the same data is often being accessed the caching aspect may not be that important.
General consensus ive been seeing recently agrees with you that you really can get it running on surprisingly low end hardware these days, and finding less than 8gb of RAM in the ddr4 or 5 era is perhaps difficult enough that my above point is moot
I did the opposite and used it as an excuse to upgrade my main PC, with the parts that got replaced being inherited by the new server.
Perhaps an unwise move due to it not being optimised for power savings, and looking at your particular use case it wouldn’t be a smart move.
Depending in where you want to have this NAS, one of the more important factors to consider is how quiet you can make it. If you only have a few HDDs they’re not too loud, but ssds are silent. It can also be worth getting some good fans and making sure you can mount them in a way that doesn’t cause unnecessary vibration to have it be real quiet.