For me, 5E only slot one per spell, whatever they like however many they like with the standard 1 in 3 chance of losing the spell. Worst case scenario, maytbe they get 6 or 7 slots. Whatever
A few extras spell slots isn't going to matter given the kind of magic items 17th and up casters probably have access too.
One of the things I've noticed is the reluctance lately for DM's to hand out items and loot. Sometimes, sarcasm here I get the impression DM's are terrified of handing out a cheap club or dagger lest it break their precious game world.
Beyond that the lack of experience most people have with high level play, me included makes DM fun at this level a lot harder
The thing is high level people are world breakers at the get go, living legends and back in BECMI they could become gods with enough experience.
If you don't like that and I get the reluctance, than set a level cap. B/X was 14, 1st edition Castles and Crusades had 10 and there are others even lower.
Otherwise be generous and let your guys be powerful.