Personally, I never use the PureOS Store. Basically it is GNOME Software, and a pretty old version of it, which hangs way too often. And even on newer versions this is not totally fixed.
I recommend the following to update: in the terminal type:
sudo apt update
The command above will check for updates, but not install them.
To install the updates, run:
sudo apt full-upgrade
Regarding the gnome-usage. I guess you see this when you open the Usage app. gnome-usage is actually the Usage app itself, so if you close it, it stops using the CPU.
Normally I use the top command to see CPU usage, but unfortunately, on the small screen of the Librem 5 it is not so useful, but you could try it in landscape mode, with the keyboard collapsed.