Why is there a G buildup? I would love imagine acceleration to be fairly constant, not to increase with time (at least once out of the atmosphere).
On the other hand you also don't want to turn your crew into pancakes or even make your upper stage look like a squashed can. You have a couple options:
- throttle the engine
- design payload weight, propellant load and trajectory to make g loads bearable till burnout
And one more thing related to stage 2 burnout - precise orbit insertion. Rocket engines used to be pretty imprecise at shutdown, delivering a slightly lower/higher impulse at shutdown at random. Due to this many older rockets have also some small & precise engines to correct the orbit insertion once the main one shuts down - many Chinese rockets have that. As far as I know, the Falcon 9 upper stage engine (Merlin 1D Vacuum) is precise enough so this is not needed.