How to prolong life of Redwood Deck

I have a 20 year old, 1000 sq. ft. redwood deck, on a sloped lot (2 to 20 feet off the ground) in the coastal Northern California (Santa Cruz) area. The deck was built using untreated 2x10 joists (probably redwood or douglas fir) without flashing or other sealant and then nailing on 2x6 redwood deck boards. In addition to "top nailing" the boards, the builder occasionally added angled "side nails" making it difficult to selectively remove rotting planks. The previous owners painted the deck (or possibly used a solid color stain), probably to combat the dark red sap stains from several overhanging redwood trees. The support structure, majority of the deck boards, and most of the joists (a few with beetle infestation or top rot) are still in decent to serviceable condition.
The deck boards were very thinly gapped, which I have tried to widen with a hand saw and/or putty knife. Even when clear of debris, many of the gaps are barely 1/16" - sufficient to let water through but not enough to permit proper air circulation. I have attempted to widen some of the gaps with a circular saw, but this a difficult task. Some of the joists, particularly where two deck boards meet, are showing signs of rot down 1-2 inches. I have reinforced some of the joists with pressure treated sisters (or soldiers as some call them). The biggest drainage and subsequent rot issues are at the outside edges of the deck where the 2x12 caps coupled with 2x10 joists prevent adequate drainage.
I see several options from here: 1) do nothing except surface cleaning and restain/paint; 2) continue to widen the gaps with the circular saw, seal/stain/treat the fresh cut sides, complete surface treatment as above; 3) remove all deck boards, repair/replace rotting joists, apply flashing (or other sealant) to joists, reattach deck boards with wider spacing, and complete surface treatment; 4) scrap the whole thing and rebuild (with Trex or equivalent). My objective is safely extract the maximum usable life out of the existing deck. Someday it will need to be replaced, but, at a cost of over $30/sq.ft., I'd like to postpone that day.
For the surface treatment, I'd really like to remove the old paint and apply a good solid color stain. However, several deck people have cautioned against the effort to fully remove the old paint/stain.
I welcome any suggestions.
Thanks, Geoff.
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In a partly similar case (cedar, not redwood, Canada, not Calif.) my painter friend turned out to be a trained deck constructor, recommended #3, completed this about three years ago (max. two days' work) and I am very happy with the results.
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Don Phillipson
Carlsbad Springs
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For a 20 year untreated deck, is time to replace. On my old deck I had flipped the boards over and extend the life for another 5 years. $30/sq.ft. seems way much.
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