Either 4s or 6s is suitable for 1:8 buggies, 4s being the sweet spot IMO. As you've found, 6s is quite a step up in power but generally costs more in the long run because you tend to break it more
Do you know what KV the motor is? Around 2000kv on 4s is my go-to combination.
Reducing pinion size or increasing spur gear teeth will reduce the top end speed but give you greater acceleration. It could also work out to be a little more reliable in the long run since you'll reduce running temps a bit.
If you want to stick with 6s you could always limit the throttle range on your transmitter? Would be a bit of a waste of the 6s power though!
Personally, i'd go 4s (either 2x2s or a single 4s brick)