This post is from Dec '22 with the latest response in April '23. The poster has surely performed his 3 pieces (it would be interesting to know how it went), but I comment because this is one of those forever topics for musicians...And it's a main interest/focus of mine. It seems many amateur/student pianists get pieces to about 75%-80% but that last 20% is like impossible. I, for one, was never taught how to "get a piece to performance level". I think the pedagogical assumption is: you practice til you're there. But I think a more mindful, intentional approach is more effective.
The tactic that has made all the difference for me, and that no one has mentioned yet: Recording. Doing a recording simulates a performance. It's one run through and the goal is "my best" (whatever that means to you). Recording has really shown me where my weak spots are - both technically and artistically. I typically go through 3 recording sessions (maybe a week apart) for a piece before I get the one I'm happy with. At that point I'm much better prepared to perform the piece live.
Prior to recording comes another step - Memorization - the goal here is not necessarily to have it perfectly memorized, but the tasks of memorization bring the musician deeper into the piece - you have to analyze the piece to memorize it, so you understand it's compositional structure better. And by getting away from the printed page, looking at the keyboard, you can focus more on sound. Some are excellent sight readers - they can read the music and play it at their best level, but I'm not one of those people.