Don't get me wrong, I also dislike minivan travel, but if you want to get around in Thailand I'm afraid it is just part of the deal if you don't want to rent a car.
If you look online you will see lots of people complaining about it and I would say that some of them have good reasons for feeling that way, but mostly it is because they are unfamiliar with the process.
The thing is, minivan travel is just a part of life over here in Thailand as well as all the countries that I have traveled to in the area. Would we all like to have our own private vehicle? Sure! This is available to you if you want that sort of thing but it is gonna cost you about 20x the price of a minivan and i don't know why but that is just the way it is.
For example: Today I went from Krabi to Phuket. This is a journey that would take about 3 hours if you had your own car. If you wanted to take a private taxi for the same journey it would cost around 3000B. My journey? 140B. (this is Thai Baht every time i type "B" by the way.)
What i am trying to say is that it isn't that bad for short journeys. In USA, I am slightly above average height and weight at 6 feet and about 190 lbs. In Thailand I am friggin huge, yet I was comfortable even in the back row of the minivan that I was on today.
the view from my quite comfortable seat in the back of my van
If i can do it at my size, there is a good chance you can do it at yours. If you can't, the private taxi is always available but if you consider that I was at my destination at 2.5 hours for 140 Baht that would have easily taken me the same amount of time had i driven my own car at a gasoline cost of at least 500 Baht..... i think the answer is clear