If you are good with a slightly jank option, I have had success with just moving the extension directory from VSCode to the VSCodium directory. Works for the Oracle SQL Developer plugin I use often. It might go against the terms in the extension, but I don’t care about that.
That doesn't help with Pylance and similar extensions. Microsoft implemented checks to verify the extension is running in VS Code, you have to manually patch them out of the bundled extension code (e.g. like this[0], though that probably doesn't work for the current versions anymore).
