I too need to "slouch" a bit more than normal to get comfortable while seated... car seat is way back, I use a standing desk at the office, I put my legs up on the desk here at home, and in my Hemlock Nessmuk XL, I have to scoot up a bit to get the Crazy Creek chair to work... If you move the thwart, that throws off your center of balance carry point, right?
I finally stopped trying to use a cross-thwart solution. I have 2 sets of thwarts; the one down by my feet, and the one my back is supposed to lean against.
I drilled 4 holes in each thwart. The ones down by my feet are about 2" apart, with roughly 12" between the two sets. The ones behind me are about 5" apart, with roughly 6" between the two sets. I then wove a bungi between them to hold my paddle pieces. You have to measure your own head, shoulders, and balance points with clamps or bungies, and then mark them before you drill.
My double paddle breaks in half. Each blade goes into one of the 5" loops, and each end goes into one of the 2" loops. Looks like this.
The knot in the rope has nothing to do with the white bungi cord... it's just a place I clip a daypack to hold it in when it's down that end of the canoe. The green pads you see are non-standard, and allow me to sit barefoot in comfort without my ankles rubbing on the bottom. The blue padding is where I set my tackle box so it doesn't make noise echoing through the bottom. and the 3x blue stacked/glued pads are my improvement on the seat; it's raised up some, for comfort.
I've since moved my 2" set another 1" in on each side, so the paddle shafts now sit right on my pack shoulder straps.