Overheats when driving?

Overheating in motion is commonly a radiator issue. They get plugged even partially and the coolant doesn't do its job. You say the heater also wasn't working, that points to the same thing. For the heater core I use CLR and a circulating pump. Basically, connect the pump, run clean water through it and drain it out. Then blow out the water and fill a bucket with CLR, let the pump pull in the CLR and circulate it through the core and back to the bucket. After 45 minutes or so I use air to push the CLR out. Then flush with clear water. Now connect it back up and top off the coolant.
For a radiator the price and time it takes to swap it out usually is nothing like the cost of a heater core, so they get swapped out. You could try a flush if you have the time. A Denso radiator with HD cooling is $120.00. It takes about 3 hours to DIY removing and replacing it. You can do it without pulling the charge out of the AC system, but it's faster if you do. You can get enough room by tilting the core out of the way.
https://www.2carpros.com/articles/how-to-replace-a-car-radiator

On most modern vehicles bypassing the heater is okay if the core is leaking. The problem with seeing clean water flow is that the radiator is a parallel flow core. As such you could have good flow across a 4-inch path in the top of the radiator, but the rest could be plugged solid. You could try using an IR thermometer to look at the core when it's warmed up, if you have a couple bands across it that were at say 190 degrees, but the rest was at 90 then you can see it's blocked. A bad head gasket would normally show itself a lot more because the combustion gas would not only heat the water but pressurize it as well. However, it won't hurt to rule that out.

That isn't a good sign, but I would verify it. Then you might want to look at blue devil sealant. I've had it work a few times as long as you follow the directions.

On the 3.6 there is a bleeder on the thermostat housing. You can see it in the attached and in the first picture you posted. The normal process is to open that with a cold engine, add coolant until it starts to run out of that, then close it and continue adding coolant until the bottle reads full cold. Now start the engine and let it run until the thermostat opens. Shut it off and let it cool down. As it cools off watch the level and add coolant to keep it at full cold. Repeat the start and run process a second time. If the coolant hits full hot you are good.

Hi,

Has the thermostat been replaced? The thermostat on this vehicle is replaced as a unit with the thermostat housing and is a simple replacement.

If you haven't, that is where I would start. I attached the directions below specific to this vehicle. Let me know if you have already done this or if replacing it helps.

Take care,

Joe

See pics below.