I have tried turning the Proxy status to DNS only, changing my SSL encryption to Full and Full (Strict), disabling Always Use HTTPS, and creating a Redirect Rule. None have worked.
The server is returning a certificate for tumblr.com and not for your domain name. This means the origin server isn’t configured to answer to the domain name. You need to set up your hosting to answer to the domain name, usually by adding a custom domain to your account there.
So I am also very much a beginner in all of this, so apologies in advance if what you’re saying takes a moment for me to understand.
This is the guide on Tumblr’s Help Center:
Domain Connection Requirements
In order for your domain to work on Tumblr, it has to meet the following requirements:
Your domain must be active. If your domain is expired or suspended at your domain registrar, the domain connection on Tumblr will not work.
Your domain’s DNS A record must point to Tumblr’s IP address 66.6.44.4. If the A record for your domain is pointing elsewhere, the DNS server will not be able to find Tumblr.
Cloudflare blocks Tumblr requests for validating the domain. To prevent connection disruption, it is recommended to have Cloudflare turned off for your domain.
I’ve done all of these, so I’m not sure what you mean by the origin server not being configured to answer to my domain name. I have also successfully connected my domain name (onelongreadingseries.com) to the Tumblr blog I am wanting it to send to. If this is not what you’re referring to by your response, could you be more explicit?