Maybe I'm missing something here, but it sounds like you can use existing ProgPro for this together with a little JS.
Create a simple field that holds whether the prospect has passed a progressive threshold: this'll be the field you set to "yes" or leave blank using Flows on the back end.
Add this field to the ProgPro section of a form followed by the additional fields you want to conditionally show, say 3 add'l fields for example. Set a maximum of 3 empty fields.
Then in JS you can tell if 3 or 4 fields total are delivered in the form sent to the browser. If only 3 are present then hide all the ProgPro fields with CSS. If 4 are delivered then you know that the threshold field was set to "yes", so you don't need to do anything special.