Getting my tire pressure right - a couple questions

18" TRD Off-Road Wheels - ATTURO TRAIL BLADE X-T LT 295/70R18

I'm researching to try and find what are the best pressures for:
1) on-road daily driving
2) off-roading

I have been driving them at about 39psi all the way around. Pretty harsh ride. For my most recent off-roading trip I aired down to about 23 PSI all the way around. Seemed pretty good for trails.

To air down I'm just using my pocket knife to release pressure and then my portable air compressor to read the PSI. It can take a few iterations to get it right, but no big deal.

But I'm getting a very different reading from the TPMS read-out in the truck vs. what I'm reading off the dial on my air compressor. Yesterday I carefully aired to 30psi front / 33psi rear using my air compressor. The TPMS is reading all over the place though (30, 26, 34, 32, etc. - it seems to vary every time I start up the truck).

Next set of questions:
1) how accurate is the TPMS supposed to be?
2) think I'm safe to rely on my air compressor's gauge?
3) do y'all use any of the fancy looking deflator valves? Saw a few folks using them this weekend and I guess it seems nice enough.