Here is one random $115 option for a thunderbolt external NVMe enclosure with seemingly good reviews: https://www.amazon.com/dp/B0BB74BQVN
Realistically, you probably don't even need a thunderbolt enclosure to have a good experience with an external Apple Photos library, but this option is still way cheaper than the prices Apple charges.
At this point I don't need it, and I prefer USB, because I don't like the idea of hot swapping something directly on the PCI bus
We don't yet know how fast the SSDs are in the new Mac mini, but the M1 Mac mini was using ~3GB/s SSDs, which is the same speed you get from PCIe 3.0, and realistically.. that's plenty fast for this discussion regardless.
