I feel your frustration! I went through the same thing while I was figuring how to work with the SNAP and get consistent power readings.
Just a side note… an FTP of 348 is pretty high. What is your cycling experience, and how big of a person are you?
Tire and pressure sound fine. Try cranking the tension knob until the spin-down time is under 15 seconds. It does make a difference.
I put a mark on the tension knob so I tighten it to the same position every time.
Only use the Wahoo App for calibration. I have never had good results performing a spin-down calibration from within Zwift.