@lawrencesystems Maybe get behind and help with code and/or sponsorship of this feature request.
https://xcp-ng.org/forum/topic/8252/tag-based-selection-for-multiple-vdi-storages/8
Cause this is a part required for what you are looking for as it will increase tag functionality. Specifically enabling tags to specify the affinity for where VDIs are stored (which SR they go to). There's already this feature present for VMs to ensure that they go to the correct hosts.