I was (almost) just that guy for one PR. Removed something like 20% or more of the codebase by leveraging the libraries and external tools we already had in use better, but it meant almost every single thing we were doing had to use the library function instead of the one we wrote. But assuming you have good regression tests and linters, so you know the code works and it's not terrible, the review should be more about overall high level quality instead of poring over every character to check correctness. It was still a pain to review, though