When you move up to 35" tires, regearing to 5.29s will slightly improve your fuel mileage and make the truck "feel" better while driving. Acceleration, shifting, RPMs, going up hills, will all perform better. Without the regear on 35" tires, the truck will be running at high RPMs the whole time and feel like it is struggling to move.
OVTune is an electronic flash of your ECM.
https://www.ovtuned.com/collections/ovtune-products/products/toyota-tacoma-2grfks-calibration
OVTune is very different from regearing, but will significantly improve all of the things I listed above for both stock tire sizes and larger tire sizes. I have done both, and in my opinion, OVTune actually makes more of a noticeable difference than regearing did. For 35" tires, I would suggest having both. If you can't afford both, although I said I noticed the improvements from the tune more, I would suggest doing the gears first. The gears are mechanical so you want to make sure your truck can "handle" the strain you are giving it with the big tires. The tune is just changing the software of your truck to improve things, so your truck won't mind if you do that later on.