Spam boarding isn't really the issue it's the low to no consequences for spam boarding.
At least against a galleon someone can do fire and/or bottom holes
but against a brig there are no consequences. Easy ship to use even with spamming boarders, easy ship to keep afloat, easy ship to speed around the sea, easy ship wreck to sloops even with very poor performance.
Probably would have to rework mast requirements to change anything. It's too easy to recover and just speed out a reset over and over. Too easy to one person a brig.
The issue is the no consequences part more than anything. Boarder spam should be a luxury of skill, not just something anyone can get away with due to imbalanced combat design.
No boat should be so powerful (super speed and very easy to do everything with) that doing everything wrong is rewarding in combat. Brig is that against sloops/solos. Galleon is not that.
As a long time solo that has fought countless brigs and galleons, if you were to ask me which I'd rather defend against, a random open crew brig with a non-new player crew or a solid galleon crew I'm picking the galleon every time. Many others would as well. Brig needs some work or everything needs some work, imo. Hopefully the game is headed in that direction, more than just the gadget stuff that has been announced.