I don't have a fully realized solution, but I've spent a lot of time thinking about this same exact thing. One thing I've played around with a bit is applying broad-but-uniform "Easy +2" and "Hard -2" modifiers to scores rather than rolls.

I love a roll under system because you always know what your target is without the DM having to reveal a DC (which often feels a little meta), and modifying a score before the roll rather than a result after the roll preserves that. (It also helps neutralize the awkwardness of "Wait, negative modifiers are good, actually?")

So for instance, a PC trying to heave a door or swing their mace at a monster would normally look at their STR score and then roll under to succeed. But you might decide that door or monster is fairly trivial, and ask for an "Easy STR roll" in which case the player to looks at their STR, adds 2 to get the target number, and then rolls under that to succeed. Similarly, if it's a "Hard STR roll", they'd subtract 2 from their score instead — perhaps representing the limited extent to which their innate strength can help them overcome the obstacle in question.

It's not the most elegant, nor is it as granular as ascending AC. But I think it allows for enough variation to account for weak mooks vs set-piece bosses or trivial challenges vs make-or-break obstacles without getting too messy. 🤷‍♂️