Yes, Chopin Etudes are brilliant even if you can only struggle through them.
I personally occasionally use Czerny Op 821 (160 8-bar exercises) suitable for around Grade 8 but like all exercises they can be made easier by playing hands separate or more slowly. They don't take long to learn and give a good general work-out when practice time is short.
Like most teachers, I give pupils Burgmuller Op 100 to study variety of touch. The notes are easy to learn (Grades 2-5) but they are much harder to play well and I usually give them to players of around Grade 6 level. (By "touch" I mean ways of connecting notes with a gesture that produces the right combination of sounds, not some magical way of playing an individual key)