Boot up for me is consistent. I don't plug my phone in to CarPlay until after the system shows the "tap screen" message is on the upper screen. Going to and from work means that I've opened the rear driver's side using the button on the door to unlock and put my briefcase behind my seat (and laid out my suit jacket if I have one that day). By the time I open the driver's door, sit down, turn on the car and buckle up, the car is booted and I can plug my phone in and things work consistently.
When I immediately get in the driver's seat, I do find myself waiting for a few seconds until the system boots up enough for me to plug the phone in.
There is a caveat ...
When I precondition the cabin using the app, the system is immediately available when I get in.
Whenever I experience oddities (reverse camera guidelines missing, collision tones muffled or absent, CarPlay doesn't automatically appear or requires me to unplug/replug the phone or defaults to the phone screen instead of the maps screen), it is a result of me driving off without a full boot up.
Now that I think about it, it also helps to have my destination preset on the Maps app on the phone and directions running (already tapped "Go" on the phone) and wait for the directions to appear on their own (i.e., don't press the screen). It also helps if I pause whatever media I'm listening to before I exit the car and then restart it manually on my next trip (assuming I'm using CarPlay media and not radio or satellite). I have the diamond button on the steering wheel set up to pause and restart media playback to aid in this.
It should go without saying that per SOP I keep salt packets in my right front pants pocket and toss a pinch over my left shoulder when approaching car unless you can see the faint image of the full lunar disk while the new crescent Moon is illuminated at which point you must resort to placing the foot of a rabbit that was killed in a cemetery by a cross-eyed person at the dark of the Moon in the passenger side cupholder, unless it it is within a fortnight of either solstice, whereupon ...