What @Ben is trying to say is that you could create a separate Activity type with different settings. You could use Gravel Ride or MTB for example and set your HR zones differently, which will influence Load. Maybe HRSS gives you something better iso Time In HR zones, but you need resting HR to use it. And you would need to log your BikePacking activities as that new Activity type.
You could also experiment with S-RPE which is Session RPE and returns a number that multiplies the number of minutes from a session with your RPE score. Or use TRIMP?
If one of those reflects better your feeling, you would still need to adjust load manually by typing that value in the Load box.