There are some things to think about and understand better with any problem. When our cars were new, they could be driven anywhere, anytime, without the concern of overheating. Sure, they could overheat, but it wasn't something to worry about. What has changed through the years?
One thing that gets overlooked is using the wrong water to dilute the coolant. If tap water is used, or even worse, well water, the total dissolved solids (TDS) in the water (calcium and silicate as well as some others) build up in the water passages, including the heat ex-changers (radiator and heater core). A 1/16 buildup is the equivalent of trying to transfer heat through a 1-inch casting. In other words, everything is going to be holding heat because it can't be transferred into the coolant efficiently and what does get absorb into the coolant can't be removed into the air going through the radiator efficiently. You combine this with a colder thermostat and not enough air flow, overheating will happen pretty easily. I know for my car; the original radiator had some clogged cores at the bottom. When your engine is running, take an inferred temperature gun and check each core. You may be some clogged and they will be cold. Also, check the temperature at the top and bottom radiator hoses. With that you'll see how much the radiator is cooling the antifreeze. Check the heater hoses also, you said running the heat helps with the overheating. You'll see how much heat the heater-core's removing.
As far as the thermostat, a hotter thermostat can help keep the engine from overheating by keeping the coolant in the radiator longer so it can cool down. Another thing that gets overlooked is the pressure cap. Make sure both seals are good and the vacuum valve in the center is working, the spring should pull it back quickly when you pull on it. For every 1 pound of pressure, it will raise the boiling point about 3 degrees. A 15-pound cap will raise the boiling point 45 degrees and a 20-pound cap would naturally be 60 degrees, but if you have a weak spot in the cooling system, more pressure will find it. Don't get confused about overheating and boiling point. If the coolant never boiled, you could still overheat the engine, but if your coolant turns into a vapor (boils), you WILL overheat the engine. The water pump has probably been changed a couple times, is it the correct one for your car? There are different impellers for different applications, the most inexpensive isn't always the best and may cause cooling problems.
Do you have an overflow tank? If not, install one. They keep the radiator full and help keep it from pulling contaminants in. I know, I know, the air will still flow in and out of that tank, but it keeps the air out of the radiator.
Something else to think about is how much heat the engine needs and how it's controlled. 31% goes out the exhaust, 8% is lost through radiation (the heat you feel when you raise the hood), the engine holds onto 30% to function properly, so that leaves 31% for the cooling system. My engine was on the dyno in 2017 and of course, they ran water through it. As soon as I got it back home, I capped the bottom hose inlet and filled it with 50/50 antifreeze, capped the top hose outlet and there it sat. I knew it was going to be a while until I installed it and I didn't want the rust to start because that will also make heat transfer more difficult.
This is just some info about the cooling system, take what you will, but your car wasn't overheating when it was new. Having a better understanding of what's going on can help solve the issue. You have a nice car, it would be awesome to enjoy it without being concerened about overheating problems!!
