Do you think an agent is going to do all of that and get you the best price/time/comfort combination for your exact preferences. Or do you think it's going to pick the first that looks reasonable? Or do you think it's going to sacrifice one dimension too much?

We already have agents for this if you really want to avoid it, they're called travel agents. They're pretty good at complex travel booking and not very expensive.

Maybe, I've never used a human travel agent. Based on my experiences with human agents in other industries (especially real estate), I think the LLM version will probably already know my preferences a good bit better than most human travel agents would bother to learn - they're infinitely patient, and not trying to maximize earnings by minimizing time spent per booking.