Turns out it is a lot more useful to (potentially) reuse the first stage than to carry a lot of additional weight to reuse the last stage.
The only non-reusable part was the external fuel tank.
But yeah, sending something as big as a bus up and down was probably the bane of it.
But to use them again a lot of efforts had to be spent, comparing to what's possible with first stages of F9. That's why Shuttle boosters are called refurbishable.