Beta readers are meant to edit your work. They can focus on different things, depending on their skill level and your needs/desires. But one thing's for sure: a good beta reader is worth their weight in gold.
There's nothing akin to the agony of editing your book. This suffering goes beyond the whole "kill your darlings" because, at least for me, I'll gladly kill my darlings if it means saving my book.
Make your setting a character.