Any way to opt out of whiteboard interviews without seeming difficult?
Offer a substitute rather than a refusal: a take-home you have already done, a walkthrough of public work, or pair debugging on their codebase. Companies that decline all three have told you something useful about how they work.
How do you estimate a task with unknown parts?
Timebox the unknown separately. Estimate the known work normally, then add an explicit investigation slot with a decision point at the end. Merging the two produces a number that is wrong in a way nobody can inspect.
Is there a simpler version that gets most of the benefit?
Yes: do the first step, skip the automation, and revisit in a month. Most of the value is in the first step, and most of the cost is in making it repeatable before you know it is right.