Wood – Which is better for deck flooring: 2×6 or 1×6 deck boards

Rubber tile or mat would be one simple approach - see duckboards for another method/approach in polymers or sometimes treated wood.

Given a solid, correctly sloped concrete substrate, pressure-treated 2x4 sleepers and a deck with no posts, etc. should work just fine - the 2x4's can even be laid flat, since the concrete deck is taking the load directly.

Paint on or pour on rubber (acrylic/polyurethane, possibly with rubber granules) coatings as for pool decks or perhaps tennis courts might also work, and might cost less.

None of the three options you've shown are likely to add substantial strength or stiffness to your floor. To add either, you'll need a single, continuous additional member to span the majority of the joist length.

The blocking is probably not critical, especially if you're doubling the joists for most of their length. It's typically installed to prevent deflection, and that's not a significant concern where you have doubled joists.

My suggestion would be to remove blocking from every second joist space and double for 80% of the joist's length in those spaces. The remaining blocking will serve to stabilize all joists.

If the original joists are sound (not showing rot), compression is probably not a concern. Even a tub of water doesn't add up to enough weight to compress healthy lumber.

Regarding your comments on the original ideas, you will see some gains if 1) you fit your additional blocking extremely well, and 2) you fasten it all together with screws and construction adhesive. The result will essentially be a load-bridging honeycomb that will improve rigidity and strength. However, the flooring already provides substantial load-sharing, so gains will be minimal.

The primary reason that it's not widely hailed is that it's a very putzy, time-intensive plan that's ultimately inferior to a proper joist sistering. You'll spend more time cutting and fitting all those blocks than you would tearing out half the blocking and slapping new lumber alongside the joists.