A repost from this post, with more clarifications;
I'd run ABP with the following adjustments so magic items still function in-universe, while not being mandatory for fighting high level foes;
(1) Item and potency bonus are separate, but only the better one applies; this also explains why that sword you looted from the stronger NPC boss suddenly drops in power in a RAW manner.
(2) Base item bonus from armour is now just "armour bonus" to avoid grave errors within (1).
(3) Each devastating attack feature lets you treat your proficient non-magical weapon and unarmed damage as something else for the purpose of overcoming damage resistance; magic, cold iron and silver, then adamantine (Monks get the same upgrades on their appropriate attack methods a few levels earlier, as usual).
I agree with the idea of, even when using ABP, allowing Potency and Striking to still continue to exist, but not allowing those bonuses to stack on top of the ABP bonuses.
I also like your suggestion and limitation that one need be proficient with the weapon to get some of the effects. I however think that that limitation should be more widespread, and not just your limitations on your extra ability granted to devastating attacks.
I'm not certain I understand, or more likely don't agree with your third part in having your attacks automatically count as different types for purposes of resistance if proficient. I feel like having those types of weapons adds flavor that this basically discourages and invalidates. Monks doing is a little different as a class ability generally on their natural attack, but having everyone do it seems like something I wouldn't generally be for, feeling like it actually would take away from the game. (but that is just my opinion)
Then magic items, would be able to get you ahead of the normal curve, but they would not hold the non-magical martial from being able to pound out significant damage at higher levels.
Honestly, I'm personally inclined to potentially use just the Devastating blows part of the ABP, but in addition to the character level requirement, also require specific proficiency ranks with the weapon for Devastating attacks; getting one extra die with Expert (& 4th level), two extra with Master proficiency (& 12th level), and 3 extra with Legendary (& 19th level). The above gives fighters a distinct advantage in the ABP, but they are the ones most dependent on its effects to be effective. The classes that don't get the highest weapon proficiencies aren't relying on the weapon die damages as much, and if a character wants to, it just means that they invest in one of the magical items, either to keep themselves up with the fighter, or take the fighter's discarded weapon rune they had to keep them ahead of the curve, when they no longer need it. If you feel barbarians and other classes have to be able to get +3 die devastating attacks you could drop the die requirements down one tier +1 die = trained, +2 dice = Expert, +3 dice = Master. I just thought there was a certain elegance to +1 = expert, +2 = master, +3 = legendary with the parallel on runes as well.
Again, I like the idea of having special material weapons in the game, so wouldn't want to try to encourage the removal of those details from the game via adding it to the ABP, as I like the flavor that they add.
Also if I was just using the devastating blows part of the ABP I'd probably allow Potency to exist, and have it be able to come from either magic or crafting quality (non-magical). Potentially on non-magical quality weapons allow devastating attacks as one skill tier above their current skill rank (so allow a masterwork battleaxe to give a barbarian with master proficiency with the axe, the ability to get the +3 dice damage at 19th level) It would be an investment, but might not need to be a high level item, as it just opens up the characters ability for its innate abilities to scale better. Magic would have the benefit of being able to be moved from one item to another via runes, thus one advantage of magic would be its portability. Quality crafted items by definition are stuck being part of the item they are, thus can only be sold to recover investment, rather than simply transferred.
Yes, I know, the issue with this is it makes the economy more complicated, it leaves the existence of these runes the ABP normally gets rid of, but reduces the 'need' for them, potentially allowing that money to be reinvested in something else later on. That is obviously what they hoped to avoid by making them not exist. But I'd rather see the variety and risk the players getting a little ahead of the curve than nerf the flavors available by saying a sword can't me magically sharper than normal, or a non-magical sword can't be better than average. Both of which are really key aspects in most medieval fantasy stories I can think of. And yes, in those same stories, normally the big hero does a whole lot more damage when they hit the beast than the struggling squire.