Having thought about this for a few more days:
1. Cantrips: You really could just remove at-will cantrips as a thing without breaking the game. It wouldn't be balanced the same (some options are weaker, some are stronger), but 5e is robust enough to still work fine with the change. How I'd suggest running it:
a. All existing cantrips are now 1st-level spells (so that they are still options, just not at-will options. Very important for Arcane Tricksters.)
b. If the cantrip scaled to character level, it now scales to slot level. (so a fire bolt cast with a 5th-level slot deals 5d10 fire damage on a hit.)
c. Remove the "cantrips" column from every class table that has it.
Optional: increase spells known to account for the lack of cantrips. I'm not certain this is necessary.
For wizards and most druids this will be a barely-noticeable change; they generally only used cantrips when they had nothing better to do anyways. Now they'll shoot a crossbow or just dodge. It's a little harder on sorcerers, but only because Quicken Spell is a lot weaker by this rule. But they can still Twin spells, so they might be fine overall.
For bards and clerics they'll just pay more attention to backup weapons. For druids who would have used shillelagh, they'll just have to decide if it's worth a spell slot. This may nerf certain very specific builds, but most druids won't really care.
Warlocks are obviously the most affected, but eldritch blast is still a thing by these rules, and really nasty when you do use it, but no longer your every-turn go-to. Melee warlocks (bladelocks, hexblades, etc) are actually even better under this rule because they didn't need at-will casting, but they can gain spell-slot boosted cantrips for heavy hits.
2. Spell Equivalency: I think this is rather baked into the system, but only as a part of attrition (and the game is all about attrition.) I'd be inclined to look for spells that should have been features, and making the changes one at a time. Probably no one will complain about Hunter's Mark as a ranger feature or bards loosing access to find steed.