Is there a way in OP to create an assignable subtask within the main task - not as a separate work package item in the list, and also not as a mere checklist.  (Hate to say "as Asana has", but yes, that's what we're trying to replace if possible.  Thank you.

You can use the option to insert a work package table in another work package. This way you'll have a work package list that you can work on (assign, change status or title, etc) within another work package. However, they will still be regular work packages.

Example: 

Find more information here.

Kind regards