I do a lot of long haul travel for work, nearly always in J (but sometimes F). My go-to outfit is a pair of merino wool leggings (lightweight or midweight, depending on the current temperatures) with a lightweight merino tool tunic and cardigan on top. Black or dark grey. Knee high boots most of the time, and I'll toss on a scarf for some flair. It's super comfortable (almost like pajamas!), the natural fibers breathe, very easy to sleep in. Once airborne and at cruising altitude, the boots get removed and replaced with a pair of washable slippers - I think they're neoprene? Also comfortable and gives me some barrier between my feet/socks and the bathroom floor. Towards the end of the flight, the slippers get removed and tucked in a plastic baggie until I can wash them.
I also will usually change out of my underwire bra into something like a camisole with stretchy support. I'm pretty busty so it's not the most flattering look, but it's a lot more comfortable for sleeping!
I also like to bring a panty liner or 2 when traveling longhair. When I don't have time to shower and change clothes before connecting, changing a panty liner helps me feel a bit fresher. (Sorry if this is TMI). I also like the travel packs of makeup removing face wipes. Nice to freshen up a bit and I've also used them to clean seat arms that felt a bit sticky.