The kitchen attached to our house has a low slope roof. We battled leaks until I gave up on the clay tiles and had a steel roof added over them. I put fiberglass insulation between the two roofs. The inside ceiling had to be replaced from water damage so we put insulation between it and the underside of the clay tiles also.
It faces south south and got very hot in season. Now it's cool and definitely waterproof.
The clay tiles on the rest of the house don't leak, but are at a much greater slope.
If I need work done on the roof from now on I'll never use clay tiles again.
Soffit vents are a great idea but forget about that silly reflective paper nonsense. Soffit vents work best if the roof has a turbine or gable vent to allow full circulation from the soffits out the vent.
The fiberglass insulation has reflective mylar on both sides, and I used two layers for a total of R52 value. The cost of such insulation (~200฿/roll) is trivial compared to installation labor.
I'm very high on insulation. It never wears out, needs no batteries, and more than pays for itself in a year or two. Best investment you can make in Thailand.