Hmm - a couple of thoughts for you Mike;
You've got deck fixings in the bow - if your not regularly needing the extra range of a second tank could these be used to secure a second tank outside of the console? - The weight in the bow may balance the second seat you mention installing?
The bow is mostly 'dead' space when underway ( ie no-one sits there) and you'd still be able to secure bags etc over the top.
In this way if you rigged the exisiting fuel line to be able to be swapped over by having a 'flying fule line' or an inline crossover switch.
I'd say any tank that fits best would work. I've never seen a portable tank with sender (within any sensible price) . My yam gauges have a display ( designed to be used with a fixed tank) - but its not connected to my 2x 25L tanks in any way . Its wired to show 'full' all the time.
I feel anything that would be able to read from both tanks ( or just the one ) or swapped over would have to be bespoke (expensive) and add another drain to (if you install one) a 12v battery that cant (?) be charged off the engine .
To be able to have a 'fill' opening and a vent and a level sender takes up space on the top of the tank - I think Space is at a premium for us smaller ribbers
Anyhow - your battery may need to be easily removed or have wiring allow a 'plug in' charger. ( not sure how you store the boat - mines a garage so this is easy in this case)
just some thoughts......