CF for SaaS, as kind of in the name (Software-as-service), is meant for hooking up your customers custom/vanity domains with your software/pointing at your origins/workers/servers.
If you wanted to let them proxy stuff through Cloudflare/use the CDN, you'd probably want the partner program/tenant setup, where they have full zones/websites and accounts: https://developers.cloudflare.com/tenant/structure/