Does anyone have an idea on how to patch the TI-84 CE's default os.
(By patching i mean decompiling and recompiling it).
I've tried opening it in Ghidra with the Z80 processor but almost no functions pop up (7 max).

The CE has an ez80 processor

Check out CEmu and the OS include file.

The chip is an ez80, which is different than the similarly named (but not as powerful) z80 processor.

Sadly, ghidra doesn't support that processor. Any other free decompilers?

It doesn't really matter, because even if you could successfully rebuild a modified OS, you couldn't cryptographically sign it so a calculator would accept it.

For reference, more people should use https://github.com/CE-Programming/zdis.

Just build it and use the example program.

So its impossible to upload the custom OS?

Yep, or people would definitely have done it by now.

Not entirely impossible; you would have to disable the boot code's OS verification code. Releasing code that does so, however, would probably force TI to make an update for the boot code and OS that breaks things. A large portion of their business model is that their calculators are usable on major exams. If people are able to send custom OSes, people can now cheat on those exams, which might lead to the CE being banned from being used in exams.

What do you want to patch the OS for? It might be possible to accomplish the same thing without any modifications to the OS by using hooks.

No particular reason why, i just wanted to see if i could (for example) edit the text in the about screen or change labels from the top of the screen.

You can edit some of the text by editing the certificate, which is way easier than modifying the OS. This only works for a limited number of strings, though.

Any documentation on how to?

Here is TLM's tutorial on how to do it, and here is the program he uses.

For other strings you can use the localization hook, though that's rather complicated to set up.