My other input is some blocks have a lot of "sand" in the coolant jackets. This sand clogs the radiator but is fluid. It's a far stretch but I did not see what engine you had or the history of that engine.

A thermostat is VERY important and also weak coolant will not carry as much heat away as 50/50 mix of good quality. Yes you can run straight water, but if it is boiling over because the temp is hitting 212, steam is a useless transmission of heat.

The proper water pump (and pulley combination) also plays into the mix. If your hoses are hard, then try it with a loose radiator cap (after it cools of course. Don't burn yourself). If the car will not keep the antifreeze mix in the radiator and violently pukes over sitting still then you have lack of heat loss issues. I.E. cavaitating water pump, blockage in radiator or blown head gasket(s) Is your water pump belt slipping?

I guess with out sounding all over the place, I kinda do that... start with the basics and work from there.

Also a post earlier asked about anything over 200degrees and old timers getting nervous. it was because the factory head gaskets and bad machining would cause leaks over 200 degrees. Honestly we have come so far in the last 40 years that we don't even remember those days. I caught the tail end of carburetors on production engine as a professional mechanic. There was a lot of those type of "rules" that are just fairy tails now. The other was a noisy rear axle... install a few banana peels to quiet it down.

Good luck with your coolant issue.
Happy Memorial day to our Vets!
Thank you for your service.
Joe