Are they in just one room? or the whole house? You like color, the vivid blue walls.

Do they darken the room?

light sand and clean. Matte poly (water base)

UNLESS you want them to golden.

I would wash them with a damp cloth, and get them good and clean, and then let them dry well, and I would also recommend the Matte poly waterbased. They are nice looking ceilings and you are right, stain would leave them a blotchy mess.

There is one spot that looks like past water marks

They are covered with water marks, or are you saying only this ceiling, and not all the other wood ceilings in the house.

Will this be a DIY project?

There is about 600sf of these pine ceilings throughout the house. the water spots are only in the one area I posted in photo. i plan on hiring a contractor, not DIY, although i like to be informed at the beginning on what im asking and expectations on how it will turn out. They do not make the rooms dark because the ceilings are high and good light from windows. the blue walls are not staying, they will be a white/nuetral. I dont love the golden orange-y color, but ive decided to try and embrace them as part of the character of the house. I just want to make them look as good as they can. thank you all for your insights!

To refresh the appearance I'd speak with a contractor about blasting them with walnut shells. I would not put water on unfinish pine ever.

Here's a room where the wood had been painted and rather than use harsh sand for blasting they used crushed walnut shells.