How can minimize the collection of personal information by mixers?
There is no ''personal'' information collected by mixers, it's just IP address and your wallet address, both of this can be used my multiple humans if they are shared.
Wallet address rarely shared among multiple user though, unless you use custodial service.
If the Bitcoin mixer uses Cloudflare they have enough information about their user saved. However, they didn't save any of their customer data theres information the hacker that access their website will get.
Can you briefly say for overall development what information can be obtained using Cloudflare?
As @LoyceV said, everything. But you should read
Mixers using cloudflare's SSL certificates if you want to know how it's possible and why Cloudflare do it.