How to stop all nodes on FCI SQL server?
So if I want to shut off my active/passive FCI SQL service, I plan to
do the following, are the steps correct?

The only steps you need are these:

Bring down FCI SQL services:

Stop Role on the role that runs my SQL FCI to prevent failover    between active/passive node.

Bring up FCI SQL Services:

Start Role on the role that run my SQL FCI.


With these you can properly manage the role services to be stopped like you need with the least impact. Once the role is stopped you can check on services.msc that SQL Server service stopped if you wanna be sure. It might take some time for that to happen because the service is going to stop gracefully, so wait until the role appears as stopped.

Another option I could think off but unsure its validity, basically
i'm stopping the SQL service and at the same time refuse it to have
the ability to failover to another node.

If you manipulate the services like this from windows services instead of using the Failover Cluster Manager, it's gonna be like fighting against the cluster: you trying to bring the services down and the cluster trying to keep them up.