Unfortunately, AOS upgrade via LCM cannot be applied per-node basis. Whenever you initiate an AOS upgrade, it automatically gets applied for all nodes present in the cluster and the AOS upgrade order (as to which node will be updated first) cannot be controlled for it.
A point to note here is that AOS upgrade is a rolling upgrade of only the CVMs while the cluster is up. Hypervisor is not restarted during AOS upgrade and so no User VMs are migrated from the hypervisor during AOS upgrades. Refer to the following guide for more details about AOS upgrade workflow - https://portal.nutanix.com/page/documents/details?targetId=Acropolis-Upgrade-Guide-v5_15:upg-upgrade-process-flow-r.html
When you mentioned about the issues with processor, I assume you meant the different CPU processor type present on your 1065-G5 nodes versus to your 1065-G6 nodes. Kindly let me know if I misunderstood. Since no user VM migration happens during AOS upgrades, I don’t believe different CPU types present on your G5 and G6 nodes should stop you from doing AOS upgrades.