I wish that list existed! You're right, there's tons. HMRC have a few test cases you have to pass in order to get recognition, but it's the tip of the iceberg. Getting recognition does not mean you're done.

What we've strived for is to make our software super defensive to input it hasn't seen before. So if you enter a tax code or national insurance category that isn't strongly defined, we error, get alerted, then jump on it (instead of falling through to some default behaviour).

One nice thing is that a few of our customers give access to their accountants, who are very on it with challenging us if anything doesn't look perfect. When we get a report, usually it's an accountant telling us ahead of time vs. a customer finding out on payday, which is a sanity check we're grateful for.