Thing is I always think I’ve found it, then they almost see they can live off me and turn into the same person. A sponge. I don’t know how I can vet that !
They aren't turning, you're just being hopelessly optimistic at the start. By your own words:
When I met him he was everything I wanted, the “alpha male” of his group,
It's not normal to think that being the "alpha male" is a good sign for a relationship. It means someone is trying to project a certain image to everyone, making them the centre of attention. It isn't always a bad thing, but is not normally perceived as an indicator of trustworthiness and reliability!
told me he’s always been the provider in his last relationship, gave his first ex 30k and last ex 10k
"Told you" is really important here. How many guys really give tens of thousands to their exes? Now filter out the ones who are discreet about it, and instead think about the ones who tell their new girlfriend about how much money they gave to their ex? Still perceiving trustworthiness and reliability?
All his mates know and can see what’s happening but don’t say nothing.
Unless you've known them for ages, his mates are his mates, not yours. Why on earth would you expect that to speak up on your behalf?
yet still manages to look good and be “that guy” when it comes to going out.
Again, projecting that "alpha male" image really obviously to other people. It absolutely isn't a strong indicator of a good reliable bet. At best, available for a shag.
I don't want to blame you for shit boyfriends, but from your own words this guy did not look like a good bet right from the start. Do you have a friend you can trust to give an early opinion? Can you write yourself a checklist to review for the next one??