There is interresting ideas in your "potential punishments/costs" section.

The reaction idea is close to one I had and i think both could work together. I was thinking of making releasing the spell on a successful strike a reaction, to save some action economy, since the Magus doesn't have AoO and the few reactions he has are very situational.
Could get a feat to use that reaction to spellstrike as an AoO too.

I never noticed the Magus didn't have many reactions until you mentioned it. If so many are suggesting doing something with the reaction, it might be the key to solving the problem.

Using reactions to manipulate the magic stored within spellstrike in different ways seems a very interesting design space. Like, expending the spell as a melee AoO, saving the spell, or... Maybe even transferring the spell to someone else for a spellstrike? Hmmmmm....

Like I said, there's plenty of fun stuff to do with Magus' spellstrike using reactions more~

WHAT I LIKE:

*Fun, Risky Take on Half-Caster*
I like that a Magus can still get up to level 9 spells but loses slots. You have incentive to make your spell slots count and make sure they're used to maximum effectiveness, which really gives a neat and fun risk factor to the Magus. Four spells did seem very limiting at first, but because I dipped more into the martial side of the Magus I think it didn't bother me. I also like that feats give abilities to lean more into the casting side or the martial side without having to multiclass. It gives versatility to the Magus and allows you to play your character the way you want which is what I've always enjoyed about Pathfinder's design philosophy. I hope that upon release we will have more feat/synthesis options that allow us to lean more in one direction or the other.

WHAT I DON'T LIKE:

*Striking Spell is too much risk and not enough reward*
Losing a spell to a missed attack is REALLY devastating and demotivating, especially given the Magus' already extremely limited slots. It's happened a lot more frequently then I'd like and I even tried to Min/Max. It really discourages use of the ability, especially when simply casting the spell as normal while peppering from afar with ranged weapons gives you a good action economy without the higher risk of losing the spell in the attack. If the intention of the ability was to make it high risk/high reward, that's fine and really interesting. However, because this is the core feature of the class, it should be made clearer (perhaps in fluff/flavor text) that this ability is meant to be high-risk. We either need a better reward or lessened punishment because a slightly better action economy doesn't feel worth the risk of losing a very limited spell slot.

Suggested Potential Rewards:
1. Modifiers to spell attack/saves when using striking spell.
2. Making double spellstrike a standard function of the ability.

Suggested Potential Punishments:
1. Having to spend a reaction to save your spell.
2. Taking damage equal to spell_level+1 as magical feedback each turn you don't expend or cancel the spell.
3. Being unable to cast a different spell until the stored one is expended (Or perhaps swapped out?).
4. An option for spell slot recovery outside of combat (or maybe another risky option to do during combat?)