On the submarine issue, I wonder if the easiest change is just making fuel non-tradable. Genuine FCs won't be affected and can retain the needed benefit of being able to fund their operations, but the more solo FCs someone has doing it, the greater the cost and larger the effort.

Put in a 30 day requirement to buy the non-tradable tanks. Or better, make all tanks in inventory dissolve if someone leaves an FC with any.

A housing primal would need to grind many millions of GC seals a day to buy all that fuel, rather than just buying up people's spare fuel cheaply and transferring it around.