ADHD

The manager's position is fallacious: you built the tool, so if you lost access to it because requirements changed or whatever... you could just build another tool

Arguably, your script is a disability aid. You've already demonstrated that if you don't have access to the tool you struggle to follow the process. They've documented this and put you on a PIP, and you're trying to address it with a change in process to suit your disability. You're doing HR's job for them even if they don't see it that way

Extreme parallel: if a person can't get around without a wheelchair, then would the manager be concerned that they wouldn't be able to do their job if they lost access to their chair? And if they are, what are they doing taking away somebody's wheelchair for?

There's a good chance you'll never be able to reason with that kind of person - management seems to select that way. But if you could, I'd point out that it's a process improvement that costs zero dollars, it's more ergonomic, and it mitigates the effect of your disability which your employer is legally mandated to accommodate.

Then again if I was quitting next week, I'd be just as likely to call the manager an idiot and go on with my day. This never helps, but if you don't mind burning bridges it's pretty cathartic