What happens after? We are all at Coruscant and we all seem to be running our own questlines that do not match up at all. My Consular friend is lvl 33 (myself and the Knight are 11),
There is your first problem - the level disparity. If you team up with your LVL 33 friend, you either won't get much XP (if you do missions for the LVL 11 characters), or you and the Knight will stand around and try not to die (if you do missions leveled for the LVL 33). I would recommend you tell the LVL 33 to roll an alt and catch up with you on Coruscant.
and he said that the class quests just seem to go on indefinitely... at least he's still running them.
Class quests go on to end game, yes.
However, all planets have planet quests that can be run by anyone. Talk to anyone with a yellow triangle over their head and they will give you their quest. You can tell the difference between a class quest and a planet quest by looking in your quest log. (They will be listed as "Class" or under the name of a planet.)
Can you ditch the class quests?
You can't abandon them. You can level to 50 without doing them, if you PvP, run FP, or do space missions. However, you won't get your companions, meaning your ability to do crafting will be reduced. You'll have to go to planets anyway to get crafting materials, though (unless you buy from the GTN).