If you are struggling to get up hills, you have incorrect gearing for 6s, your wheel size, and your weight. More voltage adds speed, not necessarily torque. It also adds significant cost.
The catch is that VESCS are not very happy at 6S because they can’t handle much current - circa 70A continuous. With 6s, geared correctly, going up hills, you will push over 100Amps on a normal car ESC. My 6s cruises up steep hills, on a 150A car esc. peaks at around 100A.
If you upgrade to 10s, you’ll need to change your gearing to match your wheel size, and motor KV. Or you’ll end up with a board geared to go 50 mph lacking real torque.