This isn't a complete solution. Rather, it is a neat hack which works on a case-to-case basis, but at least allows you to have a correct statistics on your character sheet.

Mechanically, the casting attribute works in following ways:

1) Multiclassing requirements and saving throws

2) Spell attack bonus and spell save DC

3) Warlock features such as Invocations or Patron features.

I cannot do anything with the first thing. It seems mildly annoying, but I don't think this is serious.

The third thing is probably not a problem. Many of those things are either casting-attribute-agnostic (Armor of Shadows, Fiendish Vigor, Beguiling Influence) or too complex/specific to be listed on the character sheet in more details that "as a bonus action, you can inflict a psychic damage equal to your Charisma modifier".

That leaves us with warlock spells and spell options opened via invocations.

You can circumvent it by creating a homebrew feat based on a Magic Initiate which will give you access to any two warlock cantrips and any two warlock spells of no higher than 5th level. Specify that Intelligence is a spellcasting ability for these spells.

(I named it "Bargain of the Archivist" as a homage to this epic homebrew)

So, if you really want your character sheet to show the correct attack bonus for a particular spell, you can take this feat and select this spell.

Optionally, you can emulate the effects of Agonizing Blast, since this option is available in the feat builder under Modifiers section.

Note that you, in effect, are breaking the rules regarding the number of spells known and invocations known. The gentleman's honor dictates that you should remove the appropriate number of your "native" spells so the total number is correct and balance isn't broken.

Another sad detail is that DNDB does not support "everyone get a free feat at the level 1" yet, so you will have to spend an actual feat slot on this. This is why this solution is very inelegant: you cannot take a feat for spells and a feat for Agonizing Blast. You have to combine these options in a single feat or else you're going to spend a lot of ASIs and fall behind the rest of your party.

Does anyone have any ideas how to streamline this?

Item instead of feat?  No.  Feat includes ASI. 

I'm afraid I didn't understand your reply. Could you elaborate?

Do you mean that something could be achieved with custom homebrew item, but this approach fails because items don't give ASIs? This couldn't be right.

Items don't give spells known.  Feats do.  And you can make a feat that gives you the ASI you're actually taking at that level.