I would put on a good coat of Ospho before the powder coating for additional rust protection.
Also on the heat, several companies offer small wood burning stoves for boats. Amazing how much heat energy is in wood, easy to store and will never explode, also pretty easy to find during your travels. Just a thought. I like the idea of overhead travelers! the only thing about composites is they are good up to a point and then they let go all at once and would be very difficult to fix at sea. stainless, while heavy, will bend and flex quite a ways before letting go. Also just another thought…
I’ve heard good things on the composting heads, but if your not going out for weeks at a time, I’m not sure you would really get much benefit and it is another take out and replace project, so maybe that can wait awhile, especially if your tank and pump are working well. Solar is a good bet if your keeping the boat for several years. it is getting cheaper too, but make sure you have the battery system to keep up with your needs as well.
I really like the AIS system and they now are offering a new option to AIS B+ . I’m not sure what all equipment you have and how much ocean traffic you are encountering but I would put a vote in for this system. Cheers Mark