I actually think the slider idea is a great one. How feasible it is to implement something like that, I have no clue but just spitballing, it sounds great.
The game already lets us customize our inputs, button mapping, negative edge, and shortcuts so this feels like a natural extension of that system.
Like you said, allowing us to choose the size of the buffer window would allow new players to more easily get specials and special cancels out of strings, while simultaneously allowing advanced players with no issue throwing fireballs to reliably go from crouch blocking to command normals without having to pause for fear of getting bad inputs.
In a FG where many situations are resolved within the span of 1-3 frames, it's simply inexcusable to hinder players ability to punish and/or counterpoke.
Nothing makes me want to put the pad down more than blocking a low mixup option, having a few frames to get my b1 full combo punish out, nailing it, and instead getting lanterns might and seeing Hal pick his front arm up harmlessly and stand there and say, "punch me right here in my ribs, seriously, do it....you've got about a full second to do it, so I'll wait."