I think what you're saying is that you don't want to miss any important experiences? Since most of the game content are unlocked and experienced through the main scenario, you'll be fine simply following that. The only battle content left outside of it are the mentioned Binding Coil and Crystal Tower raids which are outdated (reward wise) raid content. There are still some people doing Crystal Tower instances, so if you're interested, you can unlock it and see what the queues are like, just run through each once or something. Binding Coil has a really nice and epic story, but since people aren't really running that anymore, I'd recommend you to first level to 60 and then go back with your friend or other level 60s to see that. There are also some level 50 dungeons that are unlocked through side quests, but you will be able to unlock and run those later (with or without level sync).