> the waiter messed up because they're a jerk

That's a subordinate clause where "jerk" is referring to the waiter, not the main subject, hence it's not circular.

And people think singular "they" is OK in writing.

Try:

the waiter messed up because he's a jerk.

That's not a unique problem of they:

"The man shoved the waiter because he was in his way."

Pronouns only have a few bits of information, everything else has to be inferred from context. (A big problem in natural language processing.)

This doesn't do anything at all to resolve the ambiguity in this sentence.

"They" is generally accepted as a singular gender neutral pronoun.

And has been since the Canterbury Tales! It's a modern development that it isn't considered "correct" English.

600 years is long enough for me

Here's a short review on the use of the singular "they". It is OK. https://www.merriam-webster.com/words-at-play/singular-nonbi...

If this is such a problem, do you think it's a problem too that English doesn't distinguish between singular and plural you?

Your comment strikes me as circular.