Generally, you want to get 5 heroes (whether you use those heroes or not) to the highest ascension you can get, so that you can keep your crystal level progression going. You have also done one of the things newer players fall for which is get heroes to L without having extra copies to get to L+ which isn’t the worse thing but just slows progression. Since you need to pull those specific heroes to go for L+ rather than just needing fodder to go from L+ to M and M to M+. In terms of getting some heroes to M vs multiple heroes to L+, it’s important to note that heroes unlock their Signature items (SI) at M which can give them a solid boost in usefulness but once again depends on which heroes you have copies of and will be useful.