There is lots of great music to fill those gaps.
Baroque - Lots of movements from Bach's keyboard suites would be harder than Clementi Sonatinas and easier than the Chopin, as would many movements from Handel's keyboard suites
Classical - some of the Mozart sonatas are in that range, but also all the Sonatinas by Kuhlau, which are easier than many of the Mozart ones, and are melodic and enjoyable to play
Romantic - Schubert's Ab Impromptu Op. 142 is lovely and approachable, there are some relatively easy short pieces by Schumann that might fit the bill, as well as some of Brahms' waltzes. Have a look at Chopin's easier Mazurka's, if you want Chopin.
Post-Romantic Some of the movements from Janacek's On an Overgrown Path are not too difficult and very beautiful.
You can easily find youtube recordings of these things, so I won't add them here.