Amelia! I remember you from that other forum.

I'm hardly in your situation (and think that many people would be a bit envious of such a "problem"!), but I think I've experienced something comparable.
I've always been a good sightreader, which has meant that I've never been interested in "learning" pieces that are sufficiently uncomplicated that I can read through them fairly proficiently at first sight. That kind of material is fine for sightreading
per se, but I have to feel genuinely challenged by anything I plan to study and master over weeks and months.
The solution for me has been successively larger and more difficult projects—but that's probably true for all musicians who aspire to continue to advance technically. It seems to me this is what you need to keep doing.
Does it ever end? I've often wondered about the experience of professionals at the top of their game; if one is so proficient that the majority of the repertoire is "easy," then where's the satisfaction of pushing oneself up to the next level? We've probably all dreamed of having the sightreading skills of Liszt, but would it be gratifying to play anything at all
prima vista? I know I enjoy the process as much as the product, and that just might drain all the fun out of my musical practice.
Anyway, there's a lot of explicitly virtuoso music out there from the Romantic era that you may not have explored yet. Another possibility is 20th century music; much of what I've seen appears to have some unsurpassed difficulties. Keep in mind, too, that technical difficulty and musical difficulty may be inversely proportional. The most "simple" pieces always have interpretive demands, and effortlessly hitting all the right notes at the right time doesn't make a musical rendering. That always takes work, even if learning the notes does not.