To mitigate sag, you have to leave as much buffer room as you can between maximum actual current under load and maximum rated discharge current. Best way to do this is to use prismatic cell lipos. Otherwise building a huge parallel pack would work as well. I don’t sag much on my route 5~% and I never go too low to prolong cycle life. I put 10-19% for now, but I’m sure it’d be a lot higher if I went up insane hills.
Hubs, I imagine, would sag worse since they need more current than belt drives.