Hi Chad, you mentioned the high water temps and the engine bay heat softening the vacuum lines. Installing the coils to support the hose is really band aiding the root cause.
The heat is still the cause.
Can you run with the lower engine shield off for a test run?
Can you as mentioned above test a larger radiator?
I'm thinking the air flow through the heat exchanger and then through the radiator is not dropping the outlet temp enough.
Remember the factory WRX cooling system only has three feet of radiator hose and the 818 has what about twenty five feet of tubes?
Or the flow rate through the cooling system has slowed due to the length of tubing.