The corner designation works well to get the maximum ore without excess rock/dirt, but it bothers my OCD.
Is it possible to have an option/flag to prefer vanilla designations?

I understand what you mean. Corner designations is questionable as QoL, because it allows doing thing the player could not do himself. I’ve actually been thinking about a vanilla designations only option, but it’s super hard to auto build sensible designations with just vanilla designations. You essentially have to skip certain interior tiles or accept that it’ll not be a continuous surface. Probably it’ll lead to a lot more excavators stuck. I’m not sure how to solve it, tbh.