The approval WF that currently exists is very simple and sort of effective. It uses this multi-select field on the employee record to determine who sees the Approve button on the invoice:

The challenge I'm facing now is, how do I create a related Send Email action based on this?

The more I think about this, the worse it sounds even if I got it to work. I would potentially have an email sent to 3 or 4o or whatever people all telling them that Invoice X is ready to approve, and then one would approve it and it would already be approved by the time the others got there.

Figured it out.

Then Send Email action to the selection.

So there could be 5 (or whateveR) potential approvers, but the record creator will have to select one at the time or creation. Perhaps not ideal for if there's typically only going to be 1 approver, but at the very least, I had to solve for the approvers' supervisor being listed on ALL subsidiaries as he's the backup, so there will be at least 2 in virtually all cases.