So I'm trying to create a magical item that increases both "Spell Attack" and the "Spellcasting Ability Modifier" of a character when they equip it.  Obviously you can increase the "Spell Attack" via the bonus "Spell Attacks."  However, what I wish to do is increase the modifier that increases a spell's + bonus without increase the character's actual ability score.  I'm pretty stumped as to if I can even do such a thing.

Just to make sure I'm explaining it correctly, I'm trying to increase this:

without increasing any ability scores using a magical item.

Could someone help me with this?

There isn't a modifier for that because no official content has ever done that.

Spellcasting modifier is mainly used to determine attack and save, and there are modifiers for both of those.

Well that stinks for me, I guess.  I didn't realize the modifier was used in that way, honestly.  So if that, it would change the other two.  Welp, I do still wish I could change the modifier anyways, but it's whatever.  Thank you.

I came here to ask the same thing, I’ve manually changed both the spell attack and the save DC which works fine, but now I have to remember that my healing spells have an extra point of healing, would be nice to be able to just change it

Just increase your spellcasting ability, since that IS you spellcasting modifier. If you are a wizard for example, "spellcasting modifier" is synonymous with "intelligence modifier." It isn't supposed to be buffed separately.

This is frustrating because i want to do something similar to what the Rod of the pact keeper does.. it specifically adds a +X to spell attack mod and spell save DC

You can just do two modifiers, one for spell save dc and one for spell attacks. I’m pretty sure that’s how the rod does it, anyways.