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 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.
An addition from the comments (by Peter Cordes): An Oblex Spawn has blindsight and can squeeze through small holes, which presents some interesting scouting possibilities. And can maybe beat a single guard dog in combat. But yeah, out of place on a battlefield against something that's a threat to a mid-level party.
And another suggestion (because your DM lets your familiar take attack actions): Ask your DM what he or she is okay with... ie equipment like a plate armor and/or a shield... I know, I know an armored jelly cube sounds ridiculous. But if your party is using familiars like little sidekick warriors, I'd suggest you think about equipping it as such. Also, maybe ask your DM what you have to do to turn your familiar into an actual sidekick and let it gain some levels in Sidekick Warrior. You can find the rules for that in Tasha's Couldron of Everything p.142.
