How easy/hard might it be to find a driver for 4 days per week,

mornings- around 9:30 to travel about 20 minutes (near central rama 3-over rama 9 bridge to thonburi and costs 150 baht with tip + tollway in a normal taxi.

Why not take a taxi each day?

Because I'd like to ride in a safe vehicle with a seat belt and with a driver to whom I don't have to explain each time where to go and to ask to slow down.

I've searched online but mostly what I find are tour drivers for longer distances and to the airport.

The next time you do the journey in taxi, if you like the trip (seatbelt, driver, etc etc)

Why don't you ask the driver if he'll do the journey every day?

I'm a semi-retired uk taxi driver and the above posting is correct up to a point.I personally don't like taking on regular bookings as it ties you down to a set time everyday,and although it's guaranteed money it can cause you to miss out on substantially more lucrative work.

Nevertheless there's no harm in asking and I wish you all the best.