So we will strip first. Do we let it dry and then sand? or just sand directly after stripping.  After sanding, do you think I should do a cleaner and then brightener? or is that necessary?

https://www.deckstainhelp.com/restore-a-deck-injectable-deck-stain-stripper-review/

You cannot seal over a deck stain. All deck stains are sealers as well so no need.

Look at these brands: https://www.deckstainhelp.com/the-5-best-deck-stain-reviews-and-ratings/

       Hello, we live in Eugene, OR which gets pretty wet during the winter months. We bought a house last year and are in serious need of re-staining our deck. Our deck is rather large so some areas are full sun, but mostly partial shade. Not sure on the type of wood used, the railings are definitely pressure treated. We have mold and mildew issues on the deck. At the beginning of the year i used some random cleaner and scrubbed some of the mildew off, but it will be a problem again once it starts raining. I found the previous stain that was used, which is Behr Deck over deep base. I don't want to use Behr at all in the future after reading the information posted. The stain is wearing off of some places pretty bad, but seems okay in the areas that didn't get wet from the rains.

So question is.... Do we need to sand the deck, or will a stripper work well for what we have going on? and what type of stain do you recommend? do we need to seal too, or would an all in one work. I would like to use a darker wood stain. but not a solid color, and something that will be easiest to apply in the future. What do you recommend? Any help is appreciated.