Hi, I can concur your experience as I have had the exact same issues with PX4 v1.8.0. I am very disappointed in that it seems the rover code has been abandoned by the PX4 dronecode developers.
I was trying to implement the same set up on a Traxxas X-Maxx which appeared dead on start. Like you, I went through all Rover profile options, set the start up profile parameter number myself, I went over the set up countless times. Pixhawk would often seem to forget what it was set up as and even deleted the Rover profile form the Airframe options. I had to reinstall a fresh version of QGC to get the option back, just weird!
After trying everything, reading every topic on the Rover subject posted on the forums, begging for help from the community, rewiring and re-loading firmware and in general freaking out, thinking I had killed my rover (purchased by my company for research) I installed ArduRover and it sprang to life! It just works.
I am very disappointed not to be using the PX4 stack firmware as my flight vehicle is currently loaded and working with v.1.8.0 and my team has many work hours invested in the firmware. Upon deeper investigation we will be making the move over to ArduPilot completely going forward.
I would love to hear what the PX4 stack developers have to say and shed some light on solutions we can work with.
In order to move your project forward, I recommend you make the move to ArduRover.
Best of luck!