For #4 (strict full) the origin server doesn’t have to use a Cloudflare-supplied cert. Any cert issued by a publicly trusted CA will do. See https://developers.cloudflare.com/ssl/origin-configuration/s...
*example.com/.well-known/acme-challenge/*
Disable Security, SSL: Off, Cache Level: Bypass, Automatic HTTPS Rewrites: Off
and one big gotcha: Under SSL/TLS -> Edge Certificates -> disable Always Use HTTPS
(assuming you are using the HTTP-01 challenge).