High Availability (HA) protects against host failures by restarting VMs on other hosts, while DRS balances CPU and memory resources across hosts to optimize performance.

If the hardware version of the virtual machines is incompatible with the new vSphere environment, the VMs may not appear or may fail to register properly. Ensuring compatibility between the VM hardware version and the vSphere environment is essential for successful VM migration.
If the VMs are not correctly registered in the new vSphere environment after migration, they won't show up in the vCenter inventory. Checking the registration status helps to identify if this is the issue.

https://knowledge.broadcom.com/external/article/327008/vsan-deduplication-enabled-identifying.html

Live migration via vMotion (A), HA cluster settings (B), and DRS' automated host maintenance (C) collectively minimize service disruption. Disabling Resource Pools (D) is not typically a recommended strategy for maintenance.

If a host becomes non-compliant, the first step is to remediate it via the Host Profile. This automates reapplying configurations. Manually editing config files or reinstalling ESXi are last-resort measures.