Hey quinnmz, me and my girlfriend started the game about a month ago and have been playing as a duo exclusively thus far.
As far as we can tell, you and your friend will have no problems doing content together as a duo, whether it's leves, dungeons or quests (although certain parts of quests require you to not be in a party) it'll be a blast to do with a friend. You can link leves (i.e. work on the same leve together) if you have the same leves and if you don't you can still help out each other for a smaller reward. As for difficulty, you can adjust the difficult of leves +3/6/9 levels so make sure to make use of that.
Some dungeons may require 3-4 players (4-7 players is considered a light party) but if you are at the right level for it (i.e. not too low) you should have no problem with them minus bosses.
From what I hear, end-game in FFXIV is done with a group of 8 (also known as a full party).
If you're into gathering/crafting, being a duo won't hinder you here because more than one person can gather from a single node at a time (until exhaustion, which is when a player finishes all their attempts, however sometimes it doesn't exhaust).
And finally don't worry too much about having to commit to certain jobs, as long as you are both not tanks (Gladiators), there should really be no concern!