I'm sure you've seen this chart, but I think it's a good reference for this discussion and am posting in case some reading might not have seen it. Going from 3.3 to 4 is considerable, especially if you've been riding for a while. Three sessions per week with a longer ride is definitely good riding, but I wonder if that is enough to realize the gains you're seeking in a season? I may be misreading your post but 2x20 and 4x10 near threshold are good sessions for sure, but in thinking about this in terms of TSS, it looks like your at <300 TSS for the week, and I *think* that would need to be much higher to go from 3.3-4 in a season. I am guessing you will also need to make sure you're doing a VO2 max session once a week, as you'll also need to work on that part of the equation to get from 3.3 to 4. Maybe this is targeted in your "shorter workouts at [your] 20 min watts"?

I really like the idea of thinking about it in terms of percentages. I've had success shooting for 5% chunks.

Is it possible you have places you might be able to gain some w/kg other than through TITS (position, equipment, < lbs, etc)?