How many miles (or kms) do you need to go throttle only?
We’re all wasting time until OP answers this ‘simple’ question, IMO.
Throttle-only eats battery juice quickly. On my BBHSD I could go near full power for ~15 miles (pedaling still), or could go ride low PAS up-geared so I’m adding real human power on pas 2-3 of 9 for 20 miles and have 75% battery remaining - huge difference. On my X2 M600, I did a 30 miles (this one was very flat) ride at PAS 1/2 of 9, and still had 80% battery. I might make 10-15 miles before empty throttle only at max power.
So OP - more info is needed for people to make useful recommendations. (although I think @Timpo ’s recommendation on a dual-battery bike, even a small fat tire, could be a good answer..)