Overheating after engine, radiator, and water pump replacement

Hello,

So I have a bit of a problem. I purchased a 2010 Prius with 255,000 miles on it for dirt cheap and decided to have the engine, radiator, and water pump replaced just to start fresh. When I picked it up it was running fine, but after about 30 minutes of driving the coolant temp light started flashing so I pulled over and check the coolant level. it was low. I refilled it. I just assumed the mechanic had not properly refilled the coolant system so I decided to take it upon myself to do so. After refilling and bleeding the system the engine was still overheating. So after a couple of days I took it to a different mechanic to check to see if there were any leaks in the coolant system or if the first mechanic had improperly installed anything. They did a pressure test and determined that there was a leak coming from my resevoir tank. So I order an OEM and replace it. Refill the coolant system and start to test again. about 30 min in the temp on my OBD scanner gets up to 230 degrees, just for a few seconds, drops back down to about 210-215 and stays there. The only thing is the engine coolant temp warning light never came on. I drive it really hard to get it back up to 230, again, no engine temperature coolant warning light. Im wondering if this could be a faulty thermostat or temperature sensor giving false temp readings?

***anyways more information; yes the fans come on and the blow in the correct direction. Yes the water pump comes on and circulates the coolant (at least to the best of my knowledge it does, I don't know how to tell 100% for certain). The heater blows hot and the ac blows cold, no problems there. No leaks in any of the hoses, water pump, or radiator. The only leak that was revealed was from the reservoir and it has been replaced. No codes are coming on for anything coolant related. On hard freeway driving once the temp reads 230+ on my OBD scanner, it suddenly drops back down to the 190s and stays constant for awhile but then after about 5-10 minutes, gradually increases to around the 210s and stays there consistently unless Im constantly accelerating and pushing the engine hard.***

sorry if the info is kind of scattered but Im really running out of things to test here besides the thermostat/temp sensor. any ideas would be greatly appreciated!

 

Yes, replaced the engine and figured I could get a couple more years out of it.
I did not clean or replace the EGR. I replaced the engine because there were misfire codes and it was diagnosed as a head gasket going bad, so I decided to replace it before it bit the dust.

The thermostat and ECT sensor have not been replaced but it is what I’ll be looking at replacing next.

currently there are no codes being thrown at the moment.

The only other info I have is that there is definitely a leak in the system. There is a bracket to the right of the coolant reservoir, right below the high pressure AC line that has coolant pooled in it every time I check. The leak doesn’t seem to be coming from the reservoir because I just replaced it. But all the lines around the reservoir are dry.