Failed to mention that the VCs are all built on windows VMs and the PSCs and SQL DBs are separate windows VMs also.
When you install vcenter pointing to external PSC, it creates a machine account for that vcenter. This is only replicated to other PSCs which are in same SSO domain.
If you install new PSC as new SSO domain, you cannot point vcenter to that as it does not know the vcenter object (machine account) so this is ruled out.
You can reinstall the vcenter on same machine with same ip and fqdn however the database needs to be same as well thus you can retain inventory but vcenter uuid will change as it comes with installations.Question is why we are doing this and is it really required..
You cannot repoint vcenter to another psc in different sites is limitation from vsphere 6.5 but you can converge in 6.7 (only available in VCSA) and no need to external psc so its embedded vcenter in different site but it does not really matter as there is no external psc
Migrate to VCSA 6.7 and then converge should be helpful to move further