It occurs to me that you might do well with a simple seat on that rear rack, instead of a trailer. A couple with three young children in my circle have one on each of their bikes, and they go all over the place like that.

Regarding a single track trailer. Your wheels appear to be 20" which is typical for foldable bikes like that. My road bike wheels are 26-27" or so, and the single track connection clears it with ease. I’m sure, even with your rack, you’d have no trouble with a single track connection to the seat post. Main thing to think about would be when approaching a gradient change - assuming your seat tube is lower than mine - to go at it on an angle so the trailer doesn’t bottom out. I’ve taken mine on some light trails and have never had a problem though.

I don’t have any experience with hub drive bikes, but if you were to go with a chariot type trailer, the connection bracket could be installed on the spindle, might just need to make that smaller hole big enough to accommodate your axle.