Eric,
60 amps isn't bad for an outboard. So you can benefit from that on the way to your destination.
Definitely Class D all the way!
The ProSport 20 isn't designed to work at that level continuously.....only for a short duration in the 'bulk' charging phase. You could easily run that size of charger into the ground. So if you are going to use a charger in conjunction with a Gen as a power supply then you definitely need to step up to a higher amperage and convertor class of charger.
The Blue Sea VSR and dual circuit manual switch (or auto AC relay to replace the manual switch) is an easy layout. I like supplementing the VSR with an AC relay since the shore charger will be in frequent operation and this will make everything totally automated.

Your subwoofer(s) will be the largest part of the current draw and will be one of the initial considerations in sizing your amplifier and charging support, battery capacity, etc.
In sequence, the boat will determine the best location and orientation plus displacement for the subwoofer enclosure. And that will then determine the optimum size of subwoofer and optimum enclosure type whether bandpass, bass-reflex or air suspension. The actual subwoofer driver selection should come later. When you get a chance survey the boat and scout the possibilities.

David