I found an option that works (sort of).

I made a hidden field on my form.  I named it "user_submit_action".  Via Field Rules, it is hidden Always, and ignores data when hidden.

Then I have Javascript update the value in the user_submit_action field when the user presses a submission button.

Finally, I have Field Rule(s) that act on every single field that is required.  They set the fields to optional when the user_submit_action = Reject.

The result is the user presses any submission button, the Javascript populates the user_submit_action field with the value for that submission button.  If that value is Reject, the various required fields are changed to optional, and then the submission processes like that.  In my testing on Version 12.0.2503.10378, it worked as expected.

And since the Field rules is set to ignore the value when hidden, it doesn't save upon submission and thus future tasks all start with a clean slate in the user_submit_action field.

Here's the Javascript: 

//Add the submit action to the user_submit_action field on submission.
LFForm.onFormSubmission(async function (event) {
  await LFForm.setFieldValues({variableName: "user_submit_action"}, event.data.action.value);
});