Yes, unfortunately this is a known problem that Microsoft has yet to resolve. I didn't realize you had so many options. I know there is a limit to the number of selections, and I am pretty sure 15-20 will exceed the limit.
This probably will not be a solution for you, but I have the luxury of using Nintex forms for Office 365. With something like this I would create a SharePoint permissions group with the 15-20 directors as members. In the form, I would add a people field that is limited to the selection of people from only this group, and I would only make it editable by the manager. I would then send the manager an e-mail with instructions to open the form and update the field with the correct approver. Once a name is populated it would trigger the workflow for that approval by using a trigger condition.
Maybe you can do something similar by customizing your list form with Power Apps. If you are using the standard list form, you may be able to do this with conditional formulas: Show or hide columns in a list form | Microsoft Learn
I cannot figure out using Power Apps to customize list forms except for the very basics. So unfortunately I cannot offer any advice on that.
