I haven't worked with Cloudflare in a couple of years, but have you tried setting up origin connection over HTTPS without cert verification?

As far as I remember, there were 4 modes of connecting to Origin:

1. HTTP
2. HTTPS with certificate verification
3. HTTPS without certificate verification
4. HTTPS with "self-signed" certificate

CF always defaulted to HTTP. So you'd have to explore Origin connection settings to solve this.

Note: pretty sure last option was Enterprise-only, since it required setting up a CA and providing a certificate for establishing trust, and you can still have Let's Encrypt on Origin, but it's a bit more fiddly - you would have to set up DNS validation.