Yes it would be a huge ordeal to change like that & that's the beauty of expanding +/-0 to allow more room for magic items/feats/fighting styles/spells/etc since you don't actually need to rebalance monsters & such & can simply sprinkle in lots of little magic items*Yeah, I understand about the shrinking value of the d20. But, looking at something like d20 SW, I think 40 as the cap works fine.
You could retune ability scores back to 3-18 for a -7 to +7 with 10-11 being average, and then cap proficiency at +8-10 maybe, allowing a few extra points for magic items, features like Archery fighting style, and so on, so that (sans Expertise) the cap total should be 19-20ish.
But, you would have to retool the entire game around that, which isn't worth it IMO. So, I would just retune abilty scores to 5-15 or go with straight -5 to +5 modifiers.
.*but attune slots are woefullt inadequate for that & you need something like the return of body slots
I'm pretty sure it may have been in ad&d too but not sure how to find it