That’s how it used to work, but modern GPUs pretty much auto-overclock based on their assigned power target. Undervolting traditionally would mean less power at the same clock. But modern GPUs would just see more power available in the budget to boost clocks further instead. At least in most cases. If you go extreme enough on the undervolting it may hit a point where it won’t boost clocks anymore and actually start reducing power consumption.

Mull is no longer being maintained. IronFox seems to be the go to for mobile now and is what I use. I don’t use syncing so I can’t say for sure, but it does appear to have a sync setup in its settings.