IDK on 5e, but on 3.5e, is essential to NOT multiclass arcane casters UNLESS is a class that gives +caster level.
In 5e your caster level is some combination of their caster levels. For classes that reach lvl9 casting, you don't lose any spell slots when multiclassing. That is, if you're a Bard 17/Sorcerer 3, you count as a level 20 spellcaster since both reach lvl 9 spells.
