Majority of the things I've mentioned don't need QA. Improving performance of an existing code is fine as long as it passes the existing test suite. Dev tooling / Monitoring is for devs only. Unused code is unused, just needs review from another dev to confirm. Documentation is for devs only.

And I work on the backend in a smaller company these days. Our backend code doesn't pass through QA, we just write tests and another backend coder reviews the tests if new tests are written. QA only handles frontend.