I like to wear I really comfortable pair of pants and under them I wear mild/medium compression tights, which stop my legs from swelling, especially if pregnant when your legs are prone to swelling.
I also like to take a hydrating face mist, and i spray constantly throughout the flight! And like they say drink plenty of water..trust me it does help.

Some airlines carry pillows or cushions on board. I have seen the staff offer them to people especially on long or late flights.

Get up and walk, keep the blood flowing. One of those neck pillows in case you like to nap. Maybe other pillows for support. Not sure, don’t know how far along you are, so difficult to recommend. Maybe some herbal teas to promote relaxation

yeah maybe make sure that you get a lot of rest and keep drinking water, go for little walks to keep everything moving and if you are able to have the seat vacant next to you, take advantage of that.

Stretch your legs regularly- feet flat on the ground, then lifting your heels so your on your toes and stretching your calf muscles also helps with swollen feet and helps prevent clots as well.

Carry a pillow so if your uncomfortable pillow might help, stocking for stopping clots in legs may help I used them in hospital when I was on bed rest.

Mai sure you get up and walk every so often, also there are special tights you can wear on your legs to stop blood clots