I think the point that you are missing is that regardless if it is legal or not to construct from a 80% It must be you that does the work if you have someone or pay someone else to do it they then become the manufacturer and are responsible under the law for that receiver, hence if a suitable FFL they must serialize it and transfer through their books to you and if just an individual then in order they hand it to you it has go go via a transfer FFL hence will need a name and serial number on it.
So the to main reasons for having an 80% are now kind if dead
1, Having the satisfaction of completing your own firearm
2, Having something off the books
Factor that in with what someone will charge you for doing the work or for the transfer you may as well buy 1 if not 2 off the shelf lowers
The NV law has been challenged but as of writing this it still stands and completed receivers require serial numbers
Case details here