6 hrs is nothing for many, as said before. I’ve hauled mine in a straight load for 14-12 hrs before.
I don’t wrap anybody or tie their heads. Only exception to that is if you have a small pony who might try to turn around or bite the neighbor or something.
Bring hay from home. For 6 hours, I’d probably not even bother with water. You can try, but IME most wont drink.
I would keep the divider. I actually think it helps them as they can lean on it. Don’t put anything on the other side. It’s not safe. Trailers are built to be able to handle uneven loads.
When I get where I’m going, the older ones get a bit of bute to help with any soreness. They all get extra electrolytes and plenty of turnout. Some people do preventative ulcerguard but I don’t think it’s a great idea to give horses those meds unless they need them. Even my nervous nelly OTTB does fine with these long hauls.