Thanks for sharing @Cainuriel.
I can't clearly see the issue without looking at the folder structure, but my guess is that it's still either a version mismatch (you have to change all the @openzeppelin/contracts imports to @4.8.2), or that you're missing some files, I see you reordered the original code.
Luckily, the dependencies are cached by workspace on Remix as referred in the docs, so it may be worth starting a new workspace from scratch so you can get the newest versions.
My recommendation would be using the Clone repository option, it'll create a new Workspace from scratch, so the dependencies should match, and you can start testing or extending from there:
Best