Usually a flashing coolant light is low coolant level. If the coolant level is correct, it's often bad wires or the sender on the reservoir is bad. The last one I fixed was corrosion on the terminals in the fusebox.
Based on the information you've provided, I would suspect a failing crank pulley. You'll want to verify that isn't the case before driving much.
A simple check of the cam to crank timing will let you know. It's a relatively basic task if you can find a good DIY.
You probably know this, but once the head is off, measure all the piston protrusions and see if anything is off. If you did that last time you had the head off, you’ll have some solid info to compare to.
Good knife/cutters for the fuel lines and timing belt, cordless ratchet if you really want to hurry. It’s a 10-15 minute procedure. Fuel will leak from the pump until it’s near empty so bring a container to put it in.
Drift and big hammer if it’s ahu or 1z
What would be more useful is testing how much fuel is returning from the injectors. There is always some return flow but excessive amounts indicate worn nozzles.
That is oddly satisfying to watch though.