I've cancelled my subscription yesterday, conveniently it was about to renew at the end of the week, so I have a few days to migrate from Proton, here's what I'm thinking of, but I'd love to hear what other people are switching to
IVPN is a well-established private VPN provider, with a strong track record.
Windscribe is also of note, being part of the cross-promotional circle of agreements with other established and trusted services such as Tuta, and Addy.io
Obscura is also worth a look for the especially privacy minded approach they take to running a VPN, essentially severing the entry and exit points of your network traffic across providers so that even if Obscura or their partners were to fail a user's privacy, there wouldn't be anything lost.
Mullvad is also a longstanding and respected VPN but I believe there's been pretty severe controversy around them recently, and that should be looked into before considering them. They were essentially the gold-standard up until recently.
All of these providers have a variety of anonymous payment methods, to varying degrees of convinience and anonimity.