Yes taking another cleric slows down your wizard but it delays when a feat becomes available and what maximum spell slot i.e you won't have max level spells in the late game unless you cast from a scroll but your actual DC should be fine
Wizard can bypass such limitations. Since they can scribe to learn any spell that is up to the level of their highest spell slot. Which, if you're multiclassing into another full caster, you'll still get your full spell slots and thus can scribe level 6 Wizard spells and prepare them like any other spell.
