Can I use a CDN subdomain (such as cdn.example.com) instead of pointing my entire domain to QUIC.cloud CDN?

No. Other CDNs that allow you to use a subdomain are only using it for static content (images and other rarely-changing assets). QUIC.cloud functions as a reverse proxy in front of your entire site, and so the site itself must point to the CDN. This is what allows QUIC.cloud to reliably cache your site’s dynamic content on our global network, and to serve everything from cache in a locale-advantaged manner.