If you want to gradually move away from closed and/or paid software, afaik, the only way is through unlocking the bootloader and uninstalling programs through there.
Alternatively, there is the nuclear option, to replace the whole system, and start from zero with a distro as close as possible from AOSP. Worth noting it also requires unlocking the bootloader.
Is your drive where you install games automatically mounted by the system? In case something changed in your system, does it have the same path as Steam expects it to? And is the drive a separated storage? And though it may sound like a stupid question, I think it’s important to ask also, are you sure it’s on the storage you think it is?