DestinyDonkey I don't have any experience with using Kardia, and I don't have any devices, but I was able to create a new user profile and download the APK from a mirror online, sign up, and everything worked fine - I just don't have a device.
AFAIK, in order for apps to not work on grapheneos, the app devs themselves have to specifically know about grapheneos and essentially purposefully go out of their way to block access for grapheneos users