The most helpful tip I can give is to wait until next patch when ilvl is reset before worrying about gear.

RE: Rotation -> Depends on the fight, the more tightly tuned DPS wise the more you're going to need to perfect your rotation. A3S early prog is a solid example of requiring inhuman perfection to accomplish, but our current tier is significantly more lax and allows for much more room for error.

RE: Party comp -> This largely comes down to where your LB fits into the fight and how much of that LB do you need. A8S is an example of a fight where timing your LBs can make things significantly easier for your group in a few instances, and it's possible that doubling up might push that LB back or make it an LB1 instead of a 2 (or 2 instead of a 3). So, like rotation, it's fight dependent whether it'll actually matter.

For raiding you don't actually need to be amazing at your job, just competent. Being able to communicate, listen, and react are massively more important than skyrocketing your DPS (though being able to do both is what makes a great raider). Raiding isn't actually all that difficult unless you're leading the group, if you're just a member then you just have to show up on time and bother to try.