Render’s certificates are not something you can opt out of, unfortunately, and all traffic served publicly by your services goes through our routing tier that will always use that certificate.
I’d love to understand your use case more, though, if you are able to share some details with me. What about the Let’s Encrypt certificate is unacceptable for your third party?
In this case is the third party a identity management platform from the Dutch government, and one of their requirements is that to make use of their identity platform in our application, we must make use of the certificates that are issued by their CA (https://www.pkioverheid.nl/) to serve our web app to visitors.
Currently we already have a fix that works for now, but we preferably would like to directly use those certificates at Render if possible.