Since posting this I have submitted a feature request to Synology to solve this issue.
Wildcard User Domains
In a nutshell, Synology can check if the user has a DNS record in their domain for _acme-challenge pointing to their Synology domain via CNAME. The user has to have already have setup Synology domain under External -> DDNS. Once that is in place they can issue a wildcard certificate for the user's domain. acmesh-official/acme.sh