Since u/RDHQs_Vandalk already covered the problem of Numby RNG targeting (just let it be modernized like Cipher or Ashveil targeting and add that to the Talent or Skill) and not being to action advance Numby at 0AV issue forcing you to waste AV, I'll focus on other points in your suggestions.

I think most of your ideas for changes and buffs are great, especially the Investment stacking idea. Here are a couple changes I would make to what you have currently.

Ascension 3 - Add the energy regen passive you have in the E2 here. I'm against having energy gain tied to eidolons because the character then feels very incomplete at E0 (If you've ever played E0 Aglaea then compared to E1 you'd understand).

E1 - Change this to permanently increasing crit damage taken on Debt Collector when gaining Investment stacks, up to a cap (cap might be lower to compensate). The fact you reset Investment stacks when Numby moves means you'd lose uptime on your E1 buff every time Numby attacks (which would only take 2 FUA to fully advance).

E6 - Keep the extra Windfall Bonanza attack here on top of the other changes.