You can waste a lot of time and money relying on the factory gauge for information. You need to confirm how hot it really is. What is "normal"?
When filling the engine after you drain it, take one of the heater hoses off at the engine. Don't take it off at the firewall, that is too risky it's too easy to crack the heater core twisting on the hose. Then start filling the radiator. When coolant starts coming out of the fitting at the engine where you took the hose off, stop and put the hose back on. Then keep filling. You will still have some air in the system, but just a little bit, not the huge amount you would have otherwise.
I agree with these engines for some reason being dirty in the cooling system. My 2.9 was very dirty. It would get to the point I would have no heat at all. I would flush the system and flush the heater core, and everything would be ok for a couple of months and then it would do it again. I finally put plain water in it, and ran it a few days and then flushed, put water in it again, ran it a few days and flushed, and did this several times and finally got all the rust and sediment out. The heater worked great after that, but then I started having leaks at the freeze plugs. Finally had to pull the engine and replace the freeze plugs and everything settled down after that.