Our mIgration is now complete and we were successful!
We migrated and upgraded yesterday. In the end, the decision was made to go with 6.5 rather than 6.0 as the migration tool has been improved upon.
Details that may help others:
Our main roadblock was that the pre-migration check kept failing.
To resolve this we had to:
1) Uninstall the Veeam Enterprise Manager Plugin for VMWare
2) Because our vcenter SQL database had been internal then was externalized we had to work through: https://kb.vmware.com/s/article/2113819
( basically add a backslash)
Migration completed in just over an hour.
Post Migration Check and testing:
First we confirmed our MORefIDs were unchanged in the VCSA (we documented them beforehand)
Next, I ran a test backup and replication job that I had baselined the day before so that I could confirm post migration that CBT and the metadata repositories were still working properly.
Both jobs initially failed and simply rescanning our VCSA didn't help.
I found the fix on this forum, though not in a vcenter to VCSA migration context.vmware-vsphere-f24/after-vsphere-6-0-upgrade-remote-certificate-is-invalid-t27975.html
Basically, because our self signed cert was considered insecure (not CA issued), so we had to explicitly allow veeam to connect to the VCSA
The only other change needed, was to create a backup job fro the the VCSA per https://www.veeam.com/kb2328
DONE and thanks for the feedback!