Column Visibility based on previous people column

We've setup persistent approvals for travel requests using a Power Automate flow (not the built-in workflow), so the approved status is not lost if/when the list item is edited.

We have a choice column for Approval Request allowing "Draft" and "Submitted".

Just need a control to prevent saving the list item as "Submitted" if there is no selection in the Approver (people) column.

As you know, we can't use a people column to evaluate conditional visibility on the list form...

https://learn.microsoft.com/en-us/sharepoint/dev/declarative-customization/list-form-conditional-show-hide

And we've tried JSON formatting with a 'helper' column to show if an approver has been assigned or not:

{

"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",

"elmType": "div",

"txtContent": "=if(length([$Approver]) > 0, 'Assigned', 'Unassigned')"

}

But this doesn't set a value in the column so cannot be used for list validation settings.

To reduce complexity, we're trying to do this without using a Power Apps form and conditional visibility... but maybe it's not possible?

Already found this help redirecting to Power Apps...

https://techcommunity.microsoft.com/discussions/sharepoint_general/column-visibility-based-on-previous-multi-choice-column/3987360

Any alternative suggestions are welcomed.

Thanks!