It really depends on your use case.

Do you want to record a lot of videos with multiple cameras?
Rather take a NAS rated SSD.

How long should the data be sorted?
The longer, the more capacity you need.

How critical is the data?
If not so relevant if lost, go for a cheap solution.

I took a 256GB model that was left over after a SSD upgrade in the notebook.
It's fine for just expanding the log space of the firewall.

And I agree to the prior answer from Nalply, don't take power consuming models, like gaming SSDs. They will get too hot.

Thank you very much, I have now been able to buy a Samsung 990 Pro 2TB for CHF210, I think it will work without any problems.

Get a standard M.2 2280 NVMe from Samsung, WD, Crucial and so on. Most of them work fine despite Ubiquiti saying they support only their own. Stick with PCIe Gen 3 or Gen 4 drives. Gen 5 runs too hot for the enclosed tray. Make sure the drive has no heatsink attached. It won't fit. There's no firmware-enforced capacity limit, so perhaps up to 4 TB is fine. If a drive isn't recognized, perhaps a firmware update will fix it.

Good luck!