I wonder whether anyone has dared to try this within the EU, and whether someone has already tried to use GDPR on one of these vendors

I don’t see how they could, but in any case, it would be a nice test to see whether it’s completely pointless or actually useful.

Only for a very small percentage, and even then, only for something related to what service I actually request. E.g. they can’t just use data for marketing purposes unless I explicitly and separately opt in. But most wont interact digitally in the first place, anyway, won’t they?