On the plus side, with a desktop like this that isn't going to be moving frequently, there's really no compelling need to upgrade the internal storage. You could attach an external 1TB (or bigger!) NVMe SSD and you should be able to install macOS on that. You could just use the internal SSD as an extra storage drive. AFAIK, you're not required to use the internal SSD at all.
I've had my 2.4 TB Photos library on external solid-state storage for six years now with no issues. Four years were via a laptop and so the drive was only connected when needed (to import photos into it or to back it up). The remaining two have been with it permanently tethered to a Mac mini. Of note is that I don't use iCloud Photos, though. Not sure how macOS would handle things if I did. If you had it always connected I bet it'd be seamless. If not, probably There Be Dragons. IIRC back in the days when I didn't have my Photos drive always connected, macOS would occasionally instantiate a fresh Photos library in ~/Pictures on the internal drive for Photo Stream (which I have turned on) photos.
Big question for me is - how will it behave when you also use icloud photos as well? And what if it's an encrypted disk?
I'm asking because of the following:
Let's say your laptop is 1TB, but everything except photos and for example dropbox is under 512gb.
If you then need to restore, your new laptop has to be able to contain the full restore (1TB, or whatever is used). This also means you can't just run to the store an get a standard model (instead of built-to-order) and restore, unless you buy something that has enough storage.
Having multiple apfs volumes helps here, and I'd love to have all cloud storage off of my main one.
I’m not recommending any of this for laptop users, at all.
If the external drive won’t be connected 100% of the time, there are all sorts of headaches that can occur because Apple doesn’t really design their software to handle intermittently available disks, in my experience, except (for obvious reasons) for Time Machine.
Just have a NAS serve the files. I keep about 1/2 a 1TB collection of personal pictures and videos. When I add new folders for an event I rsync them to by NAS. If I need to I will rsync folders to my windows machine for editing (have a new 16 core 7950x I’m playing with).
Commuting by bus in not particularly fun (I assume that that is what you mean, though I'm not sure) but it wouldn't've been any more fun with a Macbook Air than a Mac mini: they weigh about the same, but if I had carried an Air I would have had to carry a charger or buy a second charger to keep at the girlfriend's house. And of course the Air costs more (even after accounting for the cost of the extra monitor, keyboard and mouse I kept at her house).
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.