If you wanted a certificate for your DuckDNS domain, it’s actually supported directly by acme.sh, lego, etc. and it looks like some people already use that approach with Synology.
If you wanted a certificate for your main domain, you could always move your main domain to one of the free DNS hosts who are easy to use with Let’s Encrypt DNS validation, like Cloudflare: DNS providers who easily integrate with Let's Encrypt DNS validation