Rereading you opening post, you cannot use the GNSS for your initial direction.
The compass will be needed, it tells you the direction on first read and you know the bearing, turn the robot in that direction.
I suggest you go and log a few walks and see how the GNSS responds (mine even smapling every second at 10mph will put me in drives or 3 or 4 meters out and that's with GPS/GLONASS/Beidou all being used on my back with line of sight to the sky. And it cuts through corners.
So log a journey, plot it and see how you can use that to get where you want. Use real data otherwise you don't actually know what you are working with.
Just like in orienteering, use the compass to get a bearing and travel along that, use GPS to know where along that bearing you are and correct as needed, you can compare after a bit RMC Track angle and there maybe another NMEA line with similar.
From you logging you will see if and when that can be used.