The tricky thing here is determining when D1 would not have any choices. As you’re, I assume, in a normal form, the previous dropdowns’ values would be sandboxed inside the form.
My suggestion is converting this to a custom form if it isn’t too much to build again.
Write the choices to user-specific columns, so you can know on the fly where choices are available or not, and structure your submission availability around that.
There would also be a case where a user enters a choice (when D1 has available options), but then previous dropdown values change, leaving D1 with no options. In such cases, I would have a live list of available options for D1, and check if the chosen value at that moment is included in the list. Only validated if it’s included.