The aft tank on my Mk2 was hopeless even after replacing the seal. I suspect someone may have given up hunting leaks on your boat and filled the tank with foam.
I replaced and/or re-bedded all the hardware on the tank and hatch. I located a split in the joint between the transom and the gutter, just where the gudgeon is fixed. I built a buoyancy testing kit as described in the Wayfarer book and used that to apply positive pressure to the tank. Liberal application of soapy water to all joints around the tank revealed bubbles in this area. Repaired with epoxy and glass tape and it now passes the test.
Threading and labelling the bags under the foredeck sounds like a good plan.
I dislike the benches. Mine were originally fixed to the fwd tank with original equipment furniture blocks! I’ve replaced those with stainless angle, through bolted and sealed with sika. The winter might see them modified to mk1 spec with the addition of a knee bonded to the hull.
I’m thinking of adding tyga ties to the floorboards in the area next to the mast to hold down water, fuel, etc. John is clearly right about never having enough of the things.