Don't overthink it. Your iPad Pro 128GB is fine. I am in the same situation, but it's more of a self-inflicted psychological torture to think about what you
might have.
Strategy wise the best approach is to wait until the price drops, then sell your 128GB (the price of that has already dropped, it won't drop as much as the 256GB over the coming weeks). Then when you buy the 256GB, your losses are minimized.
That is, until Apple launches the 512GB 16GB RAM A11X 5K OLED iPad Pro 15" a few months later. Well, I've already
shared this comic...
tl;dr - forget it and find something else to do. You will never win the game with Apple.