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.