Deck clean, strip, stain advice

Hi -

I live in NJ.  My deck is approx 14 x 28.  It is about 20-25 years old, in partial shade. In the past, I hired someone to power wash and seal the deck.  I think he used Thompsons Sealer but not 100% sure.  I'm thinking of expanding my DIY skills and cleaning (or stripping) and sealing the deck myself.

I've read tons of posts/reviews here and I'm leaning toward RAD or Defy.  I was also planning on using a transparent sealer.

Now to my questions.

Should i use a stain or a sealer? I'm just looking for protection not to change the color of the wood.

Will I have to clean (or strip) and stain every year?

Do I apply the products with a sprayer? Roller? Brush?  Any tips here would be appreciated?

As an alternative to RAD/Defy, are there any suitable products carried by stores like Home Depot/Lowes?

I powered washed the deck but I'm not sure if it is enough.  I think I might have to strip it.  I've attached some pictures so hopefully someone can give me some direction on my first (or next step)

Thanks in advance for your help!