Thanks for the tips, these are really helpful!
I'd recommend sticking with the Free Trial until you've beaten Heavensward and its post-game patches, and tried out all of the jobs. Being able to grind nearly all of them up to level 60 for free is amazing and you should take advantage of it for the jobs that appeal to you.
Glamour is honestly nothing you should worry too much about at this point. But I think some of the nicest looking gear you can get around level 50 and 60 respectively is the stuff that drops from the raids (mainly the 24 player ones). At the end of the day, it's all just superficial. Although it might be a bit hard to come by glamour prisms during the Free Trial. There is a glamour dresser which you can access at any inn. In it you get to put glamours from events and job quests for free, which you can then use to create some templates with, and this won't cost you any prisms. The level 50 job gear is honestly quite nice looking!
The game will actually inform the other players if one or more members of your party is new to a dungeon or trial, but it doesn't hurt to be social! Pulling aggro off the Tank is definitely the biggest taboo. It's one thing to pull something by accident, in which case you should run towards and not away from the Tank. But generally speaking there is a "you pull, you tank" mentality which means that any DPS (or even Healer) that decides to run ahead and pull mobs will be left to deal with them on their own. The Tank and Healer are kind of expected to be the parents of any party. They communicate and decide when and how much to pull based on how experienced they are, and the DPS is just the muscle. That doesn't mean you shouldn't do your best, but you are not the leader, or even second in command.
Cutscenes before boss fights get shorter and shorter the further into the game you get. With the exception of the final two dungeons in A Realm Reborn, which are just poorly designed but you don't have to worry about that since watching those cutscenes are mandatory. They usually just last for 15-20 seconds although it can feel like ages when you see the message about how the boss arena will be locked off popping up! Don't worry about that though. If you are not in the boss arena after the cutscene ends, you will be given the option to teleport there.
In my experience, people who start the battle while you're watching a cutscene do so because they are confident that they can manage without you for that duration (it's a bit like when someone gets booted or disconnected from a dungeon, often times there's at least one or two people in the party who are comfortable with pushing ahead while waiting for the fourth slot to be filled again). So feel free to watch those cutscenes! If someone gets mad at you for not rushing through the dungeon for their convenience they are honestly being rude. There is an option to skip any previously watched cutscenes in dungeons, which I suggest you activate for future runs.