The basic options are either finding spells/abilities with similar cast times and placing them on the same keybind for each account, or just going with castsequences. You can also put delays in a cast sequence.

Maybe something like
(Pally) /castsequence Seal of Command, Judgement of Wisdom, Next Spell, Next Spell, Next Spell
(Mage) /castrandom Fireball, Fireblast

Figure out the spells that each toon uses the most, check the casting times. Decide if you'd use them in a specific sequence each time, or if random cast order would work nearly as well. It's a lot easier to come up with a single "big-button" which can be spammed for all of your toons.

You can divide your spells/abilities into multiple buttons too. My mixed team has an opening button and a big button. The opening button is something like:

(Paladin) /castsequence Seal of Vengeance, Avenger's Shield, Judgement of Wisdom,,,
(Others) /castsequence ,,,Scorch, Scorch, Scorch
(Others) /castsequence ,,,Faerie Fire, Insect Swarm, Moonfire
(Others) /castsequence ,,,Curse of the Elements, Corruption, Immolate

On weaker mobs which will die fast, the smaller button will only be cast a few times, and then only if the Paladin needs a Seal up. The bigger button will be pushed almost exclusively, as the mob will die before dots tick and debuffs aren't required.

Against an elite mob, the smaller button will debuff the mob, and once fully debuffed, the bigger button will be employed.

Everyone has their own system that works. The fewer buttons you click the easier it is to track what you're doing. The more buttons you need to click, the more control you have over what happens and when. Find a balance that works for you.