For me the up-shifting experience have two variations (also on a G29).
- Going very slowly keeping my weight centered and when shifting it feels kind of a new regular static mount where I’m sensing the balance on the “bigger wheel” and slowly accelerating away. Feels a bit like everything happens in slow motion.
- Riding at a fast cruising speed and shifting.
1 is obviously not very stable after the shift (as speed equals stability as others have mentioned) but it’s also less scary as I can easily step of if everything goes south.
2 is stable after the shift but also with a higher risk of failure during the shift.
A few thing i did myself to help me get comfortable on my G29
- Crank length where the gear ratio matches what I can manage on a 36" (~4.0). It makes it quite boring in low gear, but manageable in high.
- Higher tire pressure for a more responsive wheel.
- A lot of body protection to overcome the initial fear factor.