Still overheats after replacing both thermostats

That sounds like you still have air in the system or a possible failed head gasket is pushing combustion gas into the system and that is causing both issues. What led you to replace the thermostats? Was it overheating before this? The system is filled by removing the cap, filling the radiator, then replace the cap and fill the reservoir to the max level. Start and run the engine until the thermostat opens. Let it cool and verify the coolant level in the reservoir. If this is the process you used it shouldn't be air in the system.
To check if it is a head gasket there are a couple methods. The easiest is to remove the radiator cap, be sure the coolant is full, then start the engine. Watch the coolant for bubbles or foaming. If you see those it's a bad sign. The next step is a pressure test on the cooling system, you can "rent" the tools from many parts stores.

https://www.2carpros.com/articles/head-gasket-blown-test