Idea: Splitwise
Splitwise is a website/app that helps people split bills really easily. Everyone contributes, records it, and you can choose to "settle up" when you want to make it even. The app will calculate for you how much you owe people, and match it to how muc you've already contributed, so there's less venmo back-and-forth nonsense.
If pooling money into a fund up-front is too much to ask, or doesn't seem like a good idea, we can each pay for group things as we go, and keep track here. Did you pay for gas for the rental car today? Did you pay for dinner? Did you pay for the Airbnb? You can record it in splitwise. And if you wanted to pay for something and treat it as a gift, then don't record it. Simple.
A possible downside is I think it splits everything evenly, so the sliding fee thing doesn't really happen here. But if this seems like the better option, we can work around that.