Well it seems to me that you can play slowly with perfection a million times over, but as soon as you play the same passage faster, you will invariably make mistakes, as all of the muscle activations are totally different for each different tempo.You cant get used to playing fast, by practising slowly and accurately.Practising slowly prepares and conditions you for playing slowly.
So to play a passage both slow and fast, you would have to practice for hours at one tempo, move the metronome up 1 beat, practice at that, etc. Thousands of repetitions, and it still might not work, because maybe there's a difference between quarter = 60 and quarter = 60.5. To move tempo from 30 to 120 will take you decades, especially if you have to move by 0.1 instead of 1 beat at a time.
And that's essentially what you are trying to accomplish with your incremental speedup approach.
I don't think this is how it works. Rather than a distinctly different technique for every speed, I think there are probably a few discrete points where you have to alter technique. Like I can walk, jog, run, or a horse can walk, trot, canter, gallop. (that's 5 I think, don't horses have to learn to canter left and right footed? can't remember)
Anyway, on that journey from 30 to 120, I suspect the technique at 30 will work fine until at some point, maybe 55, you'll come to a plateau, until you work out a way forward, which might hit another point at maybe 85-90, etc.
The key point is that if I'm right, your 120 technique works
all the way down! But you've learned your 30 speed technique so thoroughly you might never hit on the necessary advanced one.