I've been thinking about this (a lot!) I think the structural considerations would be the main obstacle in the way of getting really fast speeds. Or alternatively as soon as the prop tips became transonic you'd quickly increase drag.

I would love to spend a few months trying to build a faster machine to attempt a record. Then I'd really see what the limits are! But unfortunately I have to work to earn money to live 😀

I am kind of thinking about making a follow up video to answer a couple of questions, like what determines the maximum speed and what would happen if the wind went to zero after the cart was already moving. Together with a better model/simulation to show exactly what's going on. Let's see if there is sustained interest in the topic. This last video took a few weeks of solid effort to make so there needs to be a lot of interest to make it worthwhile.