You are right; I didn’t factor in RPi boot time to that figure. I’d guess maybe 60-90 seconds total? By the time I’ve turned the ignition, backed out of the driveway, and put it in drive I have a GPS lock.
You can make a Pi boot much faster than that. The main thing to disable is wait for network available. Other embedded boards I've used eg Rock960 are ssh-able within about 20 seconds or less.
I’ve been watching ChrisFix and Scotty Kilmer in Youtube and while they don’t agree on much both say you should wait a few seconds after starting the car before driving away.
Something about engine oil and lubrications. Probably doesn’t harm anything in the short term but if you have time, probably not a bad idea.
Edit - much much faster (4 secs), actually: http://himeshp.blogspot.com/2018/08/fast-boot-with-raspberry...