GSM dongles simply aren't that popular anymore.
Wifi access is near-universal, so the average consumer will pretty much never be without internet access - and their phone is a pretty suitable backup hotspot.
The people who do need GSM-based internet - like surveyors or door-to-door people - are probably better served by tablets anyways, and definitely don't want to deal with a USB dongle. If they have to, they'll just get a laptop with a M.2 GSM card. Whatever remains probably doesn't need 5G speeds to begin with.
So yeah, it's a tiny market. Expect them to be expensive. There's a 5G USB dongle made by Apal available for €400, but that's about it.
Are you sure 4G won't be enough? You can get one of those for €50 or so.