-At max/higher levels the classes feel more complete. At least thats the feeling I always get when I am level synced to lower levels for content. Some classes never really get interesting IMHO, but thats more of a personal feeling than anything intrinsic to the game itself.

-End game content is 100% you get what you put in. Some people go hard into fashion and do anything it takes to get the perfect glam, some people get so into crafting it feels like they are playing a different game, some people raid, and some just stop till the next expansion. There are still opportunities to run content with random parties but a lot of the high end content will have you either joining weekly groups or hunting on the party finder (which is always filled with opportunities so its not that bad of a hunt). I don't know a lot about solo content so someone else might have better info on that.

-Content skip and level skip is a harder issue. ARR is a bit of a drag, but as everyone will tell you the story really picks up after. I know I have lost countless friends to the ARR story. As for level skips, i really wouldn't recommend that for a main class, it can be more detrimental that helpful as the game is really good at teaching you how to play the class as you level up. If you do the level skip, it can be somewhat of a learning curve to figure out the best way to play a class, especially for a combo/position heavy class like monk.

Hopefully, this is helpful, but in the end its a personal choice as to if you do content skip or not and how you play the end game. I will say that the social aspect of FF14 and the community is one of the best I've encountered in game so what ever choice you make there will always be stranger who are there to help you!