A grand mystery, hybrid system and precollison malfunction

In November my 2017 Prius Prime l, 225k miles, began having issues.

Order of operations:

1. November 2023 - precollison and hybrid system malfunction light come on seemingly at random. Paired with this is an inability to select eco or sport mode or cruise control.

2. Shortly after the grille shutter light came on, and I confirmed that it was at least stuck in the open position and did not have anything stuck in it.

3. December 2023 - Car begins behaving strangely, and has a bucking feeling when switching from ev to ice.

4. Shortly after the hybrid system malfunction now adds power reduction, and the engine seems to rev up at random while driving and experiencing power loss at the same time, goes back to normal if I give it more gas

5. I change the spark plugs, and notice one of them was much less used. Bucking stops but precollisoon and hybrid system issues persist.

6. I then notice the coolant is low, and steam coming from the exhaust. I confirm that it is the head gasket. I run stop leak, and in my ignorance put it through both reservoirs. The stop leak was unsuccessful anyway.

7. Given the insane price of having the headgasket done, I opted to have the engine replaced for 500 dollars more with a used 32k mile motor. They also replaced the hybrid drive water pump as it was siezed, apparently because of the stop leak. On the way home after this the precollision and hybrid system malfunction lights came on with reduced power output, the inability to use cruise control or other drive modes, and random engine reving. The problem only seems to occur after being on the highway for a while. If I shut the car off and on again then stay at surface street speeds it still shows the malfunction lights but without the power reduction and everything else is functional.

I am 8k dollars deep at this point and I'm hoping that I will be able to fix the remaining issues by myself. Any ideas?