Good practices for patching SQL nodes in High Availability solution
Approaching this by thinking about each SQL instance separately.
Patching SQLDR\Inst_B
This is a secondary replica, so it can be patched/rebooted independently.

Switch SQLDR\Inst_B to be an ASYNC replica (if it's not already)
Patch SQLDB\Inst_B and reboot (windows patches, SQL patches, etc.)
Verify SQLDR comes back up and the AG is healthy.
Switch back to SYNC if required.

Notes:  While Inst_B is offline, the T-Logs on Inst_A will be kept around to replay when Inst_B returns.  I wouldn't recommend patching Inst_B while doing index maintenance or large data loads on Inst_A.  If Inst_B is down for a long period of time (think days) then you'll probably want to break the AG replica.
Patching SQLFCIPrimary\Inst_A

Patch passive node and reboot.
Failover to Passive node.  This will cause your (hopefully brief, e.g. &lt; 1 min) outage.
Confirm AG is healthy
Patch new passive node and reboot.
Failback if required.

Notes:  Since you are using MANUAL failover, there is no risk that you'll inadvertently failover to Inst_B.
This is the simplest way to accomplish regular patching, and just needs one or two FCI failovers.  There is no reason to stop WSFC roles.
Whether you do Inst_A or Inst_B first doesn't matter, but I'm risk adverse, so I'd start with Inst_B.