Weight limits on backless boosters are sort of....eh. They're there because...no one's sure why
In a crash, the seatbelt and the seatbelt alone is supporting the weight of the passenger, not the booster.
All in all, it's YOUR decision, but I'm more worried about a good belt fit than I am about a very arbitrary weight limit on a backless booster.
I mentioned the Harmony booster because it fits adult butts so well
I believe it has a 110# weight limit. If I were in your shoes, I would be perfectly comfortable using it.