On your synology holdout: I run two of them plus a custom truenas, and I treat the storage layer as the deliberate boundary of the declarative model rather than fighting to pull it in. Topology is truenas on fast ssd backing the running vms, synology one as primary, synology two as the backup target for both truenas and the primary. The synologys are imperative islands — dsm doesn't want to be config-managed — so I codify the consumption side (the exports, and the vms that mount them) and treat the boxes themselves as data, not infra. Truenas is the exception, since the api gets you closer to declarable.
Are you leaving the synology fully hands-off, or driving any of it through the api?
1. backup, an RS422+ (w/ 2GB): 4x8TB RAID6
2. storage, RS818+ (16GB): 4x12TB RAID0 (everything not easily replaced is backed up)
3. san, an RS2418+ (64GB): 8x1TB SSD RAID10, and 4x5TB RAID10
I can't picture too much of a reason to drive it through the API, or at least not yet - still getting it all finalized. But I also managed to figure out IPv6 with Xfinity across my VLANs and SLAAC.