I would go for the small horse over a smaller animal. I can do more things with it, riding and driving. Do you plan to mostly show, or more a “little bit of everything” using this animal?
Maybe when I get quite old, a pony will do, 12h range, Welsh or Welsh cross. Sturdy, with substantial body and big gaits to do distances with. I had such a pony as a kid, I did everything with her rIding and driving. Rode her 6 miles to the shows, competed, rode her 6 miles home in a day.
For now though, 14h to even 14.2h is a great size animal. And you can get substantial without being bulky. I always think of having to cool equines fast doing CDE things with limited cooling times. Those very deep bodied animals have a harder time cooling their body core. If you plan to ship this animal to Hawaii, it needs to be able to cool easily in the warm temps. The big pony, small horse can manage a rider easily, as well as you adding a passenger to your vehicle fo a long drive. Minis will work hard, yet they just can’t go fast or handle loads the weigh more than they do except for short bursts, maybe in the ring. The cuteness factor is huge, but I want an animal who can do distance in shorter times.
And what is this reaching over the pony to harness? New to me!! Does not sound safe, you are not able to view his entire side to know you got things attached correctly on the offside. Do you walk around him before or after hitching to see how things look? This before getting in to drive? I have made mistakes, missed a crucial strap even working on both sides of a horse!! Caught it before getting on the carriage with the final walk, visual check, around horse.
Lots of nice animals around that drive well. Perhaps more common here in the Midwest, where we have a lot of equines of all kinds. With driving animals being in demand for show and recreation, so trainers keep producing them. Small Morgans could be just the ticket, they can be large pony to small horse, already trained to drive. They love a job, being busy. I would stay away from the hair breeds, lot of upkeep and skin issues in warm, wet climates.