With a stock spur on a T4 the rule of thumb would be to start with a pinion that roughly matches the turns of your brushed motor (double for brushless). If you 'tre running a 19T, start with a 19T or 20T pinion, for a 15T, use a 15-17T pinion.
I would try charging your LiPo at lower than 1C. That charger can only charge at up to 5 amps max, so its at its limit and will likely false peak unless you use a DC power source. I've had better luck at 4.5 amps when using AC power. The vision peak ultra doesn't read total charge, instead it offers total time (minutes) of charging as its measurement.
Also, make sure you have used the yellow button to select LiPo as the battery type. It's not spmart enough to know the difference, so will attempt to charge it as a NiMH or NiCd if that's what's selected. Neither of those choices will properly charge a LiPo as the LiPo charge sequence drops the amperage toward the end of the cycle repeatedly until the pack is completely full.
GV2 Pro, XXX4G+, Blitz SCT, CRC GenXL