If you really want something upgradeable, used enterprise SFF is the way to go: https://discountelectronics.com/

However, the hardware market is in a weird spot right now; you’ll get far more bang for your buck with an Intel N150. You can find a 16GB DDR5 w/ 1 TB SSD around the $200 mark, and that’s what I’d roll with in your shoes, assuming you don’t mind living without a spinning disk. Your Jellyfin and Immich instances will run far smoother.

This is the way.

Depending on the nature of the sim, it could probably even be done with ~80 GB or less of existing SSD space using zram w/ zstd.