Normally, familiars have around 1 or 2 HP... because the normal familiar options are not that good.

The best way to defend your familiar is mentioned in the description of [*find familiar*][1] itself:

> As an action, you can temporarily dismiss your familiar. It disappears into a pocket dimension where it awaits your summons. Alternatively, you can dismiss it forever. As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 30 feet of you.

The familiar is not meant to deal damage. It's designed to help you using the Help action, or to let you cast *cure wounds* at range. It's a utility tool and not a weapon.

That out of the way: I think *mage armor* is already a good deal. And I wouldn't spend more spell slots on a familiar. If you need to save it from an otherwise lethal situation, just dismiss it.
TBH you already have a pretty sturdy familiar... normally they're much frailer. That's why the most familiar owners have an animal that can fly or at least climb, to do scouting or get into places you wouldn't get otherwise.

  [1]: https://www.dndbeyond.com/spells/find-familiar