I very much recommend some of the easier Polonaises such as the posth. G minor and B flat major (I think?) ones. I also recommend the first two Op. 69 waltzes and obviously the posth. A minor waltz that everyone plays.
The very first piece I learned by Chopin was his prelude in E Minor. I would say this is a fairly tricky grade 6 piece. Next his Prelude in D Flat, the Raindrop. A gorgeous grace 7 piece which could be learned with some patience. His E Flat Nocturne is actually fairly simple and I would say it is grade 6/7 when it comes to difficulty. A little trickier of you want a bit of a challenge is his B flat minor Nocturne which is absolutely stunning. Worth having a look at a couple of his easier Mazurkas (f major perhaps) and there’s also a couple slower waltzes which should be fairly simple. If you are a competent grade 6 pianist then you should be able to have a look through these all and see how you get on!