For the ship combat, you get more stuff from boarding, I’d say it’s mostly worth boarding the haulers/transports, there is a counter in the top right of your screen during the boarding process which tells you how many enemies you/your crew has to kill. Upgrading your boarding crew helps with this a lot.
I also agree the xp gain is slow, I do wish you would get it from combat, farming and building as well but I can see how they have intentionally slowed it down to match your exploration which makes sense, it’s just annoying 😂