Update: I now noticed that my Qubes 4.0 installation on another SSD doesn’t seem to be affected by those freezes (at least I didn’t get one all day). Both setups are basically identical (BTRFS file system; running on exactly the same PC; …). Only difference seems to be the type of SSD. I’m not aware that anything else is different.
With this SSD I’m experiencing the freezes: Samsung 870 EVO 4TB
With this SSD I didn’t experience the freezes yet: Samsung 860 EVO 2TB
I’ll continue testing this assumption.