For cleaning there are brush attachments for power washers.
Maybe a rental store or big box rental? Or just buy it and use whenever.
There might be ones separately powered.
That is a big job but you'll save money DIY.
Not sure about cleaners to use. Citrus based sounds about right. Something safe for grass and pool.
Once you get back to raw base material it should be sealed up.
A search brought back a few brands: glaze guard, 511, ole, and many others.
There are gloss and matte finishes.
For later maintenance there are anti-mildew sprays. Someone recommended wet-n-forget but there are others..