[MENTION=23716]Gadget[/MENTION] has it: you should definitely try your idea before you add it permanently, and few options are as permanent as a feat. A gadget (heh) with non-replenishable charges is a much better start.
[MENTION=6866167]Thurmas[/MENTION] has also an important point - whatever you do, keep it simple! ☺
That said, my focus is on explaining why NPC epic casters can do it.
So neither an item nor a feat cuts it for me. Take Vizeran from OotA for example - had he become involved in combat, I would definitely had him break most restrictions on Wizards.
But npcs generally don't take feats. I feel a feat solution isn't the true 5e way. Class features, on the other hand, are often replicated in monster stat blocks.
Re: an item - It makes the Archmage much less impressive if he or she must rely on a doodad. And items can be looted. And having NPCs rely on items that then make the party even more powerful is exactly what's wrong with d20...
At that point I thought "okay, so an Archmage prestige class!"
But that is pretty freaking far from keeping it simple...

So in the end I settled for repurposing Spell Mastery.
It achieves everything I need.
Yes, it means that if you expect a lot of play at level 20, you're better off playing a wizard than a cleric or sorcerer.
But then again, if you expect to play at that level much, 5e really falls apart in general and you need an epic manual.
I am mostly thinking about level 10-15 heroes talking to (and possibly fighting) level 20 NPCs. I really think that's the top end of what the current rules support in actual play. Basically it boils down to the Archmage NPC state block, which is simplified to the point of being anemic. A repurposed Spell Mastery, allowing "dual concentration" goes a long way of rectifying that
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