Likewise, I use the same taxi company for every trip:-
https://www.thailandtaxitransfer.com
I have found them to be excellent on every trip. I use WhatsApp to contact them (or the occasional email). The boss, Chok responds very promptly. I have used them for BKK-JOM-BKK and also for longer journeys from BKK to Saraburi. Even arranged a 6 person VIP van a couple of weeks ago to transport the family from Saraburi to Jomtien for a mini break.
The cars are immaculate and the drivers speak perfect English. They always send a message around 30 minutes before departure to notify you that they are on their way or already waiting at departure point. I also found their prices to be very competitive after having a look at a few firms. In two instances they were circa 30% cheaper than Nam's taxis. Also provide female drivers if preferred.
Like @VBF there is no financial benefit to me sharing this contact. I just find them to be a very reliable company.