Nice... but yet again there's no indication that the OP is speaking of anything other than the built-in Progressive Profiling behavior.
Don't get why this thread has attracted so many custom suggestions given the pretty clearly stated need?
Maybe just lots of people wanting to be helpful. Not sure I see the problem ¯\_(ツ)_/¯
Problem comes when people don't know how Progressive Profiling works, where it works (everywhere), or that it even exists as an option. Since there are ostensibly knowledgeable agencies still saying it doesn't work with embedded forms (confusing it with Pre-Fill) there's a definite knowledge gap.
So of course advanced, JS-driven behaviors are very exciting, but it's a question of firming up the base of knowledge.