You don't have to use cloudflare's dns to make use of the proxy (CDN, waf, ddos protection etc) see https://developers.cloudflare.com/dns/zone-setups/partial-se...
> If you are on a Free or Pro plan, full setup is the only one available. This is the recommended and most common option.
That means to use this partial setup you need to be on at least the Business plan, which is $200/month when billed anually ($250 monthly).