One bit of problem is that guidelines are subjective rather than objective. The example models are literally contest winners so they already have been judged to be exceptional by subjective opinion of judges. The positive language in guidelines encourages submission leaving judging quality to the user. There are also suggestions for interaction, promotion, maintenance and updates - out of which I think interaction and promotion can be quantified.
I’d recommend you to tighten down the language of the guidelines and requirements making them less subjective and more objective. Good example of objective guideline is how you clearly defined that render/screenshot cannot be cover image. Make sure to add examples of models that fit the guidelines and models that would be of decent quality but not fit for exclusive program.
For interaction and promotion - you have data on users interaction in comments/rating, so based on interaction history you can auto-reject users that aren’t interacting on minimal level (and you have to define that level) on previous submissions (at least in a timeframe… I guess for people with loads of models over time constant interaction with users would take way too much time so it has to be objective but achievable). You also could request users add social media links and proofs if you’d want to go that way but that might discourage some people (or some people don’t use social media) so that point about promotion would have to be reworded. Currently there’s little way for creators to promote on MakerWorld itself.
Regarding licencing for exclusive models - I think you are a bit too lenient allowing models which switched licences. It creates a burden for later. You should just require the model is “standard file licence” from the get go and clearly require that the model is not hosted outside of MW in publicly accessible way (and thus non-exclusive)