First off, have you removed the rad cap with the engine cool and verified that the coolant is full? Sometimes overflow tanks don’t refill the system like they are supposed to.
Replacing a t-stat is never a bad idea, but it is probably not the problem in this case. The op says the car o/heats on the highway at high speed. This rules out the fan, as the fan is not needed at speed.
There is a possibility that the water pump is the cause of the problem, but don’t bet on it. For this to be a water pump related issue, the impeller of the pump would have to corroded enough that the pump no longer moved enough water to the radiator for cooling. This is very uncommon.
The single most likely cause of your issue is the radiator is plugged either internally or externally. Over time, dirt, pet hair*, and other items can plug the airflow though the radiator. No airflow = no cooling. This shows up first when the heat load is the greatest. IOW on the highway at high speed. The cure for this is to clean the radiator and AC condenser. You can use a water hose, and blast high pressure water from behind the radiator to clear the air passages, or you can remove the radiator and holding it front side down drop it on the ground from about knee high. You will be amazed at the pile of dirt left on the ground after doing this. You will still need to clean the AC condenser.
for internal plugging, the radiator needs to be removed and sent to a radiator shop for cleaning. The shop will remove the tanks and run a rod though the core to clean out the deposits that accumulate in the passages over time. This is far superior to any flush that you might add to the radiator, and has the bonus of not being an environmental nightmare.
If you want to go with the take no prisoners get it done right the first time approach do this:
Drain the coolant and remove the radiator.
Dispose of the used coolant properly
Take the radiator to a shop and have them rod it out.
While the car is at the shop, take a high pressure water hose and clean out the AC condenser so it is clear.
Change the stat if you have not already done so.
Reinstall the radiator, and install the correct mixture of the correct coolant (they aren’t all the same!)
*People that keep large hairy dogs in their garage with their cars can get a blanket of pet fur on the front of their radiator/AC condenser.