No, you can set up paid memberships with any available payment provider you have enabled in your module payments....

Allthough stripe is a good choice concerning 'disaggio' and some kind of 'open source in mind...'

Thanks @Peter . So when exactly is the Stripe Connect needed?

Stripe connect is needed, when You want to use stripe as payment provider.

Product -> payment -> site action

Selling membership -> any provider -> upgrade done by UNA or manually by yourself (depending on how you recieved the payment)