Which 2018-2020 sets are good for roof parts? What 2018-2020 sets are best for roof parts, such as corner slopes, 2x2 slopes, 2x4 slopes, etc.?
I would suggest to look into buying individual parts on Bricklink in desired color, shape and amount. Any other set you can buy most likely won't have all the parts you need and you are left with otherwise unnecessary elements.
If you really need a set with roof specific parts, the set I could think of now is Emmet's Dream House/Rescue Rocket (70831). Selection of slopes isn't ideal and color (blue) may not fit your need, however the amount of specific items is substantial compared to other sets. Newbury Haunted High School (70425) could also be a candidate for some number of slopes, but it is still rather limited.
Alternatively, you could look into creating your roof without using slopes. Take a looks at sets like 31078, 75954, 10267. You can build your roof with plates and tiles if used wisely.