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.
You can try to get them from LEGO pick-a-brick searching for "roof" gives you quite a collection, but misses the ends and corners that are available, so add another search for "45°" to get them.
Combine this with Bricklink and buy whereever it is cheaper or where you can find the wanted quantities of desired elements.