We have about 4000 square feet of wooden wraparound porch at our hunting lodge in E TX and it's in need of some work. I'm planning to put my boys to work and just straw boss things a bit.

The closeup shows some of the mold and mildew residue that's in various places. We also have some water spots and oil spots (4 wheeler, griddle and grill) in some locations.

So I'm thinking power wash, sanding, and some water seal.

Do I need to treat the mildew and oil spots with something before pressure washing? Should I put some type of detergent in the pressure washer?

I'm thinking sand the worse spots with an electric palm sander, then pressure wash, then lightly sand all the surface area, then blow the dust off and seal.

I'm not sure if doing the light sanding of all the wood is overkill, but I want it to look fairly uniform.

Any input or thoughts would be appreciated.

You are asking Top Drawer, Primo care advice that others will post later.

This is food for thought and might guide you towards later choices that may save the backaches.
Once had high-IQ clients, (e.g. PBS fans, hospital/schools admin, inner circle society, ex-CIA, built personal aircraft from kit, etc.) that owned a cedar siding 2-story house & auxiliary buildings on grounds. Obviously DIYers, they found an acid wood stain they could bug-pump spray on very old cedar siding that "ate" the dirty-brown & light-green (in your photos) then turned it all a golden honey color with a well sealed appearance. Of course, anything metal like window frames had to be taped off for protection.

The stain brand was Flood but the version above may have disappeared in the decades past when employed. I'm sure the advances in paint/stain with Devoe, Ben.Moore, even Behr leading the way can offer something superior and more to suit a refined taste.