Having played an artificer in a game that used the spell points system from the DMG, I have to say that helps a lot with flexibility. With spell points, you can provide a mechanism for full caster reserves and just allowing complete upcasting (spell levels up to class level/2) would go a long way.
I say combine the "mystic arcanum" with the vancian approach of having the Artificer pre-cast them. Call them Exemplars, make them Rituals, taking +10min and then you get a "spell grenade" you can hand off. Downside, requires time & prep. Upside, the Artificer can give someone else a single charge wand of banishment proton pack. Its like a junior Spell Glyph without the ruby material component and it can be moved. Very much in keeping with the Artificer motif.
Yea. That's how I'd do it.
You can't cast Disintegration (or any other spell) directly. Instead you make a Disintegration Gun (or any other spell).
And you have some points to recharge items.
Also some bonus for concentration. Like the item needs to be attacked, not the user. Or maybe it just last the full duration no matter what.
Also, i would eiminated any temporary magic items. You can just make a sword imbued with Magic Weapon spell, armor loaded with Shield of Faith, and a Shield loaded with Shield, boots with Longstrider, and can hand it to the fighter.
Downside is much less "spells" prepared. Say 1 per level.
Upside is more "slots" and flexible upcasting, and being able to hand things off.
Level 1:
Spell Storing Item: Choose 2 cantrips and 1 level 1 spell from the Artificer slell list and craft an item that stores that spell. Each spell must be placed in a different item. The item can look however you like, but it is obvious that the item has been modified to do something. You can replace 1 item with a different one after a long rests.
You can activate the item with the spells normal casting time. All spells have a material components of the item, and must have either a verbal or somatic component your choice for each item irrespective of the spell. Spells with concentration last for their full duration or until the item it is destroyed and use your DC or casting modifier.
Durring a short or long rest, you may also instruct an ally on how to use the items. They can then activate it as you could. Attempting to activate an item without instructions takes 10 minutes of studying to make an Arcane check, with a DC equal to your spell DC.
Infusion Points: You gain infusion points equal to your Artificer level. Durring a Short Rest, you can touch one of your expended Spell Storing Items and expend a number of points equal to tbe spell level in order to recharge the item.
After a long rest, you recharge all your Spell Storing Items and regain all your Infusion Points. You regain half your infusion points durring a short rest (minimum 1).
Level 2: Bonus to item crafting. (I.e. Craft half the time with tool proficiency, double the time without tool proficiency).
Level 5.
Quick Infusion: You can recharge an Spell Storing Item with a level 5 or lower spell as a Bonus Action.
Item Drain: You can spend an Action to drain one of your Spell Storing Items and regain a number of Infusion Points equal to the spells level, up to your maximum.
Level 7: attune to 5 magic items
Level 11 After a long rest, you can add Contingency to one of your Spell Storing Items. You do not need to pay the material cost.
Level 14: attune to 6 magic items.
Level 17 ???