Everything was definitely up and running by about 5 seconds before the GPS unit started returning data.
Maybe just unplug and replug the connector, and try
GPS_GNSS_MODE,7 or 67
whichever gives you lowest most reliable HDOP.
It also doesnt hurt to set
BRD_BOOT_DELAY,3000
to allow everything to settle before the flight controller boots up. I do this with all flight controllers now even if they dont have CAN devices, since sometimes just plugging in the battery would cause a bit of craziness.