I don't hate the idea of raising the speed caps in general, and I certainly agree that many-minute travel times aren't strictly
necessary to convey a sense of scale, but I have to disagree with OP's approach. It's not just the scale of space, but the
range of scales that is problematic. When you have a setting where some destinations are a
million times farther away than others, only an instant "fast travel" system is going to make all travel times reasonable. In particular, if you set the acceleration to cap the travel time to the most remote destinations, the far more common close destinations are going to feel too close and some will be actively harder to reach because they flash past right after system entry.
That said, long AFK times
are poor UX design.
I think back to playing Witcher 3 recently. Not uncommonly, if going somewhere I haven't been yet, it's 10+ minutes of game time navigating my way there, and that's galloping on horseback. But there's abundant scenery to look at, various dangers lurking in the forest, and I did have to do some active navigation. That was fun design. The challenge with Elite is that space is both large and empty, so you can't simply implement the same thing. But for instance: instead of scenery, we have instruments. Let us use the FSS while in motion, handing over guidance to a crewmate or the SC assist, and give us more elements like that to interact with.
There's a well understood principle of paths of least energy for orbital transfer.
Those paths are non-linear.
Implement them in the game and have them provide a reasonable boost to SC acceleration when following them.
That provides a non-AFK method for those in a hurry while not affecting those who use that SC travel time to do other things.
Least energy orbits don't apply to supercruise, because our motion isn't remotely Newtonian. But I would love to have a skill-based component that would let you get places faster through skillful piloting, and something thematically similar to "slingshot" maneuvers would be a good way to do it. Another idea would be to have the fuel scooping zone do double duty as a SC boost zone, where by making a few passes around the star or other massive body you
increase the acceleration achieved after breaking free.