Nothing like a good old BKK transportation conversation. If you ask five "experts", you will get five different answers and none are really wrong. As far as the op's specific question. I used Bolt last time, I am adventurous that way. It was not that great of an experience, it was getting the pickup location coordinated. My wife is originally from Thailand, familiar with where we were and it was a pain to get the rookie drive to the right place. I only mention that as that is somewhat the same experience as Grab. Now, I have not used the designated Grab pickup location, that should help a lot. You can actually pick your vehicle in Grab and a lot of SUV's lately seem to be available. With regards to getting a private car. That is more straight forward but unless you pay extra to have someone actually meet you, it will be a person that works for multiple companies. You meet that person and they call the driver and then someone escorts you to the car. Personally, I find that annoying as I have waited a bit and I could have delt with the location circus with the Grab driver in that amount of time. That is just me though.

If you are not familiar with the BKK arrival experience, I would recommend the private car method that you mention. It should be easier as the car service minders are all right outside the immigration/customs exit.