The Exiles set (36 dungeon) will make it easier to progress to 45, and you can certainly use it until you get your 45 set. I know I did. It may not be the most time efficient however, attempting to get the full set. It's definitely not required.
I've mostly done the party route of trying to party everyone in the nearby vicinity. Sometimes people flat out refuse and it becomes a game of trying to tag everything in the area in a passive aggressive show off. And sometimes people are already in a party.
You don't have to have an awkward leave, though most will just leave as they're done. In the Korean version, and during this past alpha, I thanked everyone for the party, and usually got a thanks back (in the Korean server - West so far seems to be rather stoic and mute). Sometimes I ask how many people have left and I'll go ahead and help them. If they leave, they leave. If I need to leave, I say thanks and leave.
Unfortunately this is something most MMOs do. I think GW2 was probably the most popular example a game changing it up and promoting individualized loot and getting rid of mob tagging. I highly doubt this would change, as it's been a part of the game for years now.
I found the game to be quite fun while I was playing the Korean version (I believe the OBT). The combat was really fluid. Especially with how everything appeared to be lagless. It was great to play a Korean game that didn't visually feel like you were stuttering. Of course, there was obviously some, just not noticeable with how they implemented the combat. They definitely did housing better than ArcheAge, way better. I don't know how their open world housing would have been, but after the drama with land in ArcheAge, I think it was a good move to change it.
I spent most of my later time playing just going on my boat to fish in the ocean, and breeding horses though. So I can't speak to much end game (I stopped around level 40).