How can I be specific about which CDB I want to connect to wrapping a PDB?
Quite easily.

The full syntax for Easy Connect is:

Understanding the Easy Connect Naming Method

CONNECT username@[//]host[:port][/service_name][:server][/instance_name]


You just need to specify the service_name and the instance_name as well.

$ lsnrctl services
...
Service "pdb1" has 2 instance(s).
  Instance "RAIN", status READY, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:3 refused:0 state:ready
         LOCAL SERVER
  Instance "WIND", status READY, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:3 refused:0 state:ready
         LOCAL SERVER


You will have more than 1 instance serving the same service. The below may connect to any of the instances, but the listener decides which instance it forwards to your request, not you:

sqlplus user/password@localhost:1521/pdb1


The below commands always connect to just one of the instances. If that instance is down, the connection attempt fails, even though there is another instance providing the same service:

sqlplus user/password@localhost:1521/pdb1/rain
sqlplus user/password@localhost:1521/pdb1/wind