Which class has the easiest rotation throughout the MSQ?

[Shurrikhan]

Until 50 and again at 56, wherein you gain 2nd and 3rd positionals, Dragoon is incredibly simple to level. The tanks are easier still, but Paladin gradually falls under the dps of Warrior and Dark Knight in the majority of open world mob fights and receives its basic dps filler combo only at level 60.

For the UI, I'd recommend just keeping your screen as clean as possible. You can use pop up set-ups via macros, e.g. "/hotbar show 7" - "/hotbar show 10" to quickly switch things out. Unless already used to otherwise, I'd keep your cooldowns, player buffs/debuffs, and target pane close together.

Though difficult to manage perfectly across all jobs, you can even set all your abilities to, say, bars 1-3, and then place just the abilities you need to see on bars 4-6, and swap back and forth between them. Either shuffle the binds for those core skills to the 4-6, or just memorize their positions. If using a controller, the cross-bar already saves a good deal of space, and I'd just place the core skills in an easy to see area (with the status effects and target hud), with the actual cross-hotbar to the side.

You can also close your chat panel when not using it for even more space, and downsize all elements to as small as you can still comfortably enough see them.

Don't bother with macros except to allow for better Netflix n' Craft. They only check once per second, so you loose 20% or more attack speed for using them, given the sub-2.5s GCD and a 3-second counter. Even if you get below a 2-second GCD, where the desync would be minimal, they still often mess up.