> The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.

Now, I'm not saying that directly applies to your problems, but there's good reason this is a saying.

Going from 80% to 100% will take as long or longer as going from 0 to 80, because the last 20% is all edge cases that are a nightmare to test and mentally exhausting to comprehend.

Sometimes, it's just never cost effective to go from 80 to 100, and it's best to just leave it as is, even if there is some manual work left over.