I have no idea why your Krita is blank there… I use Linux Mint too, and never had this issue. Though my Mint is probably a bit older now. Have you tried the appimage? The appimage has all patched dependencies instead of using the system ones, so it sometimes omits some issues on the system.
And the ppa is so old, it might even has the older resources system, so whether that one works or not, it doesn’t really matter in this case.
Can you please check the appimage and see whether it works and whether you see all the brushes despite the warning at the startup?
EDIT: it could be some kind of graphics drivers problem. Do you use Wayland? And what if you change the settings in Configure Krita → Display → Canvas Acceleration?