more, please

Can someone recommend another sf author besides Asimov and Heinlein that is just as good (and similar to either)?

Arthur C. Clarke and Hal Clement are both good authors that wrote similar works.

The Foundation trilogy itself was later extended by other hands. So those authors (Greg Benford, David Brin) would be good places to start.

Similarly, Spider Robinson was heavily influenced by Heinlein, and he was entrusted with finishing one of RAH's manuscripts for publication.

And John Scalzi's Old Man's War was pretty much of a direct reply to Starship Troopers.

The Foundation trilogy itself was later extended by other hands. So those authors (Gregory Benford, David Brin) would be good places to start.

Similarly, Spider Robinson was heavily influenced by Heinlein, and he was entrusted with finishing one of RAH's manuscripts for publication.

And John Scalzi's Old Man's War was pretty much of a direct reply to Starship Troopers.

The original foundation universe by Asimov is only 14 volumes long.
I decided to read everything from the beginning, it was a great surprise to see so many works that would fit between his books.
Search for the Isaac Asimov's, Robot city series:

1. Odyssey by Michael P. Kube-McDowell (1987)
2. Suspicion by Mike McQuay (1987)
3. Cyborg by William F. Wu (1987)
4. Prodigy by Arthur Byron Cover (1988)
5. Refuge by Rob Chilson (1988)
6. Perihelion by William F. Wu (1988)

When you are done with that ... look for the "Robots in Time" and then "Robots and Aliens" (12 books more), If you are a Robot fan (like me) you might also enjoy War with the robots.
Along these lines you might also like:
Mirage, Chimera, Aurora, these three books are all from Mark w. Tiedemann
and also the Caliban Trilogy from Roger McBride Allen.
Enjoy!!