Unfortunately, there is no straightforward way to achieve your goal!
You have two options:
Boring Manually Option
Manually Save all Projects Schedule and Upload it to the destination Project Server 2016 farm using Microsoft Project.
Then Import and export the Projects Subsites using PowerShell.
Migrate Project Content Database from ...
Unfortunately, There is no direct way to apply the newly created List workflow on the current site to the corresponding list in other subsites!!
Export the created workflow and manually import it the corresponding list in other subsites!! I know it's a big headache but it considered a workaround solution.
Create a reusable workflow On the Issue ...
The +ADD TASK & EDIT LIST option is only available with SharePoint Task List Project Type.
In case, the Project Type is Enterprise Project, this option will be missing because you must add the project tasks via the Project Schedule PDP as shown below. (Or via the Microsoft Project).
Check also additional reasons at Missing the "ADD TASK - EDIT LIST" ...