I don't think zero unit tests is the right answer either. And if you actually take the time to read all 300 and cull the useless or overlapping ones, you've invested much more than 10% of the time it would have taken you.
Having a zillion unit tests (of questionable quality) is a huge pita when you try to refactor.
When I am writing unit tests (or other tests), I'm thinking about all the time I'll save by catching bugs early -- either as I write the test or in the future as regressions crop up. So to place too much importance on the amount of time invested now is missing the point, and makes me think that person is just going through the motions. Of course if I'm writing throwaway code or a POC, I'll probably skip writing tests at all.
In order to add coverage for scenarios that I haven't even thought of, I prefer fuzz testing. Then I get a lot more than 2-300 tests and I don't even pretend to spend time reviewing the tests until they fail.
If you want to use an LLM to help expedite the typing of tests you have thought of, fine. If you just tell it to write the suite for itself, that's equivalent to hiring a (mediocre to bad) new grad and forcing them to write tests for you. If that's as good of an outcome as doing it yourself, I can only assume you are brand new to software engineering.
The main benefit of writing tests is that is forces the developer to think about what they just wrote and what it is supposed to do. I often will find bugs while writing tests.
I've worked on projects with 2,000+ unit tests that are essentially useless, often fail when nothing is wrong, and rarely detect actual bugs. It is absolutely worse than having 0 tests. This is common when developers write tests to satisfy code coverage metrics, instead of in an effort to make sure their code works properly.
200 decent unit tests are better than zero unit tests.