The target audience of final smashes in their current form and a roman cancel-esque mechanic (or most of the other common suggestions for meter mechanics) are pretty much entirely separate. So long as final smashes are like, um, that, I don't think an expansion to the meter is worth the effort.
Now, if final smashes were all reworked significantly? That's another story. But it would be a massive, fundamental change to the game. But also probably too big of a change to be relegated to a toggle. I think it's either go all the way and make it a core mechanic, or nothing at all. Like you can look at squad strike and how little attention that gets (an occasional side event that probably most people forget about), and then figure how much balance and design and mechanical work has to go into making all the meter stuff, and then what's the point? possibly i'm sounding too negative here because I actually think proper meter mechanics would be super cool for smash and has been probably the single thing i've wanted most since well before ultimate. But if it's a toggle then I don't see any chance for it to get turned on.
Basically I'm still salty about what happened with custom specials lol
re: inputs I don't see as a particular problem since smash has 2 or 3 wasted buttons on default controls (2 on gamecube, 3 on switch), so barring a separation of short/full hops and shielding and parries I think you can just take one of those buttons and make it a super button (which would be combined with other inputs for whatever different functions). And if the gcc ever dies somehow then that's a third button free for anything. There's just no need for multiple shield, jump, and grab buttons right now.
And on balance in the context of the original suggestion: I don't think it matters much. Final smashes are already as unbalanced as it's ever going to get.