As
@indivision says, the domain registrar doesn't need to be the same as the DNS host; you can leave your domains with OVH and move DNS to Cloudflare if you like--though you may also want to move the registration to Cloudflare where it's likely less expensive.
If you are unable or unwilling to do that, another option would be to use another ACME client of your choosing to get the cert (I like acme.sh), and use either my Python deploy script, or the shell deploy script that ships with it. Either of those scripts should be able to deploy the new cert to your SCALE system.