Regardless of whether or not you provide your own SSL certificates, cloudflare still uses their own between their servers and client browsers. So any SSL encrypted traffic is unencrypted at their end before being re-encrypted with your certificate. How can such an entity be trusted?
CF is not using „their own“! The certificates the client see must be provided and authorized by the provider of the service. Or put in other words: CF is acting as the hosting provider to the outside, to the clients.
The rest of journey is „inside“ the domain of the provider of the service. It is totally normal that traffic has some journey to go and often it never touches the premises of the provider or even a server owned by the provider.
The important thing that all the part which from a customer‘s view is „internal to the provider of the service“ (behind the CF address) is responsibility of the provider of the service, no matter what 3rd party services they use.