Welcome, @rschnat. As has been said, the amber repo is now superseded by byzantium, introducing a lot more stability, dependability, and new features. I don’t know what is going on with your PureOS Software store (maybe time and date not set correctly on the device?), but in any case, it’s more desirable to upgrade at this point, which should fix everything for you.
You will need to do this on a computer running some recent version of Ubuntu (Mint, etc.), Debian, or PureOS. If you don’t have that already, you’ll have to make a bootable USB of one of those distros to complete the upgrade process. (That’s trivial to create, as you probably know from your Mint experience.)
Here are simplified instructions, no frills, that you can follow once you have the live USB booted: Time to flash is now - but how? - #16 by amarok
That will take care of all the dependencies. BTW, in the case of the Librem 5, it only creates problems to try what you did, i.e. changing “amber” to “byzantium” in the software sources. Unfortunately. Sure, it would be nice if there were an “easy button” for the upgrade, but sadly there isn’t yet.
The next repo version (crimson) is here for desktop PureOS, but is not ready for the Librem 5, due to the added complexity of adapting the phone functionalities, Purism says. Let’s hope they will be able to make a simpler upgrade path when it’s ready. (No ETA on that yet.)
Once you’re running byzantium, I don’t think you’ll have that store issue anymore. It still might take a few seconds or a minute for everything to populate, as it does on desktop. Familiarize yourself with the preferences and the search function there.
You might also like some of the apps posted in this long-running topic: List of Apps that fit and function well [Post them here.]
If you run into issues during the reflash, post questions in the forum and someone will be glad to help, I’m sure.
EDIT: And if I’m not mistaken, flathub is included by default in byzantium.