Hi,
This is too confusing, especially without the flow. The description doesn't make sense.
Once I save the populated template, i.e. generated document with a unique ID, I start and wait for approval.
Ok so you should have the File name, File Path here in this action since you just saved it.
Once approval is received, I need to make further changes to the previously generated document.
Ok, so open the same file, get the content, make your changes(you should have the path here since you created it above)
The issue is that since I saved it with a unique ID, I can't tell a new Populate a Word Template action a particular existing address from which it can pull fillable fields.
What does giving it a unique name have to do with the fact you said YOUR code saved it, so why don't you have the path?????? You had to give one to save it.
I can provide the filename as a variable, but since the file doesn't exist yet, or is not specific enough, Power Automate can not pull specific fields.
This makes no sense to me at all. How can the file not exist, you said you created it in step 1
I would recommend however, that after you create your file, before you kick off your approval, you put a Delay action in, just to make sure the file is created and opening/changable later in your flow.
To me I am confused, and do not understand the problem without the flow, but feel what I said would help.