It won't make a difference what cert you use for 2 reasons:
Your current setting of Full does not validate a certificate issuer. It just checks to see if the server is accepting requests over SSL (meaning port 443). So it does not matter if you use Let's Encrypt, RapidSSL, or even if you generate your own self-signed cert. The issuer is not checked at all.
Even if you were to change from Full to Strict, both Let's Encrypt AND RapidSSL are valid certificate authorities (CA). Cloudflare trusts certs issued by those CA's so it would continue to work with either even if you were to use the more secure/strict certificate validation process.