CrimsonCarcharodon
Explorer
Have you considered only giving your spellcasters access to spells if you already know how they work? Instead of inventing new monster abilities out of whole cloth, you just need to remember how fire bolt and burning hands work. You don't even really need to track spell slots, unless you expect combat to last more than three rounds.
That's the easiest way to do it, yeah. For most cantrips, I just put it down as an attack, same way I would on my own character sheet.
Sometimes, though, I really like having crazy 'spells' that are different enough so that the players don't go "oh, it's just a fireball. 8d6 damage, we can handle that." Same reason I don't color code my dragons by alignment, really.