Rpi 4b in to Topping D10 for car audio (airplay)

[Miss Sissy Princess]

Are you sure that your power supply is adequate for the RPi plus DAC and is not dropping below 5V during boot? In addition, if the RPi USB cannot supply sufficient power to the DAC you may need to use a powered USB hub.

I think you're on the right track except for the Pi4 USB power capabilities. The USB power goes straight from power supply input of the Pi4 to the USB jacks.  The D10 is pretty low power (it's a DAC only, no headphone amp), so it shouldn't be a problem.

Less likely, but still possible explanations could be a noisy and/or poorly regulated power supply or a USB power cable is too small a gauge and/or too long, leading to excessive voltage drop due to cable resistance.

I'd drag an extension cord out to the car, use the official AC power supply, and verify that everything works that way.  If it does, then it's most likely what you said; the car power adapter.  I'd look at options from Anker.  I like the fact that they seem to design for safe failure modes (as opposed to some of the no-name Chinese adapters that fail by dumping ~13.7-14.7V going out to a 5V device).