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.
I was really looking forward to thunderbolt accessoires, until I saw the prices. An external bay for a GPU (just a piece of junk metal!!) was over $400 when I was looking at it.
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
No, it wouldn't. Thunderbolt versions 3 and 4 are giving you 4 lanes of PCIe 3.0 on an external connector. You can easily get 3GB/s from an NVMe SSD in a Thunderbolt enclosure, which is the same performance as putting a PCIe 3.0 SSD into your computer.
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.
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.