[B. W.]

Don't give up on your temp gun yet! I use mine almost daily (I'm a mechanic) If you measure right at your thermostat you can actually "watch" the Tstat open & close (pretty cool) In so doing you can verify it is working & it's opening at the correct temp. Don't get too picky about exact numbers, if a 180° opens at 185° it doesn't matter, close enough.

to confirm your gauge is accurate you need to shoot the sender in the driver's head. From what you posted it's close enough.

So, from what you've posted, your engine & your gauge are doing pretty much what you thought all along. This is good to know.

My opinion (FWIW, everyone else has given theirs!) you've covered all the basics, some more than once. The one item you have not addressed is the radiator. You can look down the tank & it "looks" good but you cannot see down the tubes & these are what get clogged.

Take your new gun & shoot the radiator starting at the driver's top (hot coolant in) then shoot various points to the pass side bottom ( cooled coolant out) In perfect world, you want to see about a 50° drop across the radiator.

Time for a radiator (easy for me to spend your $ but it's still cheaper than an engine!)

side note: 50/50 mix (or any ratio) versus straight water will not make any difference. From the tech articles I've read, water with a "wetting" solution removes heat the best BUT the difference IN A PROPERLY OPERATING SYSTEM is negligable.