I ended up buying a couple of these from Illumn
(though they shipped only one of them, so someone’s likely asleep in their shipping department)
Xtar PB2S QC 3.0 + PD 3.0 Portable Li-ion Charger and Powerbank
They take 21700 unprotected cells, which I also bought, and Illumn assured me this powerbank has overcharge/discharge protection.
As others have noted above, the round cells seem more reliable than anything with flat-bag cells.
I’ve had one of those flat cells balloon, in a cheap little powerbank, in my own experience, and I find self-disassembling batteries scary.