At present, summoning a <=10 HP creature with stats equating to a level 1-2 combatant has far too niche of an application to hog a prep slot and be restricted to one per short rest.
Speaking of which, Wizards should be able to cast ritual spells without having to prepare them (
wizard class features). But with BG3 allowing you to swap prepared spells at any time you can summon your familiar and then swap the spell for something more useful
