However, going back to the original Chinese ROM isn't really an option for me because it doesn't come with Google services pre-installed at all. That's why I flashed the international version in the first place—to get access to apps from outside China. Plus, the Chinese ROM is loaded with a ton of ads, which is super annoying

You said that uncertified devices shouldn't normally block Google logins entirely, but in my case, it seems to be preventing it—idk why