No it’s that you’re reading into Blackbeard too much. Oda develops characters for the sake of developing them. The fact is Blackbeards narrative ends at the one piece, but Luffys narrative goes beyond it.
The one piece won’t be the end of the manga, Luffy will defeat Imu after it
Pointing out extremely obvious facets of Blackbeard is not reading too much into anything. Blackbeard is collecting powers. He's chasing the One Piece. He wants - according to his own subordinates - the world itself.
Oda revealed another figure failed to challenge for control of the world in the past, a figure that rivalled Roger and counted Emperors among his subordinates - to whom Blackbeard is
also connected.
Reading into things too much is how you arrive on ideas like Sabo leaving the Revolutionaries to join the Strawhats. I consider your take on Blackbeard a similar misreading.
We're not going to learn the secrets of Blackbeard's past so he can not be the final big bad, and don't think for a second Oda doesn't know what he has achieved with Blackbeard as a writer.