If your request takes longer than a hundred seconds and you cant split it into smaller chunks your only choice is to unproxy the host by switching to .
Assuming that you still have a valid certificate on your server it should not affect HTTPS in any way.
That is a bad choice as your site is not secure in that way. Only the connection between the user and Cloudflare is encrypted, everything else is plain text and so effectively not encrypted. I can only strongly suggest to install a certificate on your server.