How does it work for you in double thirds or legato melodic octaves? Assuming it works okay, why are you sticking with such easy repertoire alone? Assuming it doesn't, have you just decided never to transcend your current level? If you wanted to go further, it just might turn out that your knuckles aren't doing better than either my own or the concert performer I worked with, after all. Even if you don't, look to what an artist like Cortot could do with a tiny miniature and ask yourself what it takes to bring out that level of sound production. If you have wonderful control that leaves nothing to be desired, you must make some uploads to the audition room.
My legato 3rd and octaves? Thanks for asking. Legato 3rds are indeed where I found it exteremely beneficial to be able to control the knuckles. Just a year ago I was unable to play them without lifting some fingers early, but almost overnight they became confortable to excecute and now I wonder how I ever found them so difficult. That's fascinating in piano playing for me, how I can suddenly be able to do something that seemed impossible for so long.
I still haven't been able to solve the problems with my RH octaves, so melodic octaves obviously won't work really. Left hand I'd say they are quite ok. I have a long 4th so it helps.
Technically my biggest problem is still the RH thumb. I see a little improvement, but not enough. My RH shape is completely off in anything over a 7th on white keys because I just struggle to reach the notes. My thumb joint starts acting up really easily. Arpeggios is another problem area, because I cannot get my hand to a comfortable octave, I cannot properly take advantage of grouping.
Why do I play the pieces I do? Because I like them and I want to play them. Difficulty or levels have no value for me. Since I am a very slow memorizer, I prefer to play short pieces. Some of the pieces I have studied actually are technically rather demanding, especially for someone with hand span issues. Advanced repertoire tends to have RH octaves/large chords everywhere and I have never felt really comfortable about changing the original to better suit my hands, so I avoid those pieces until (hopefully) one day my hand has adjusted. I might just play something more "advanced" in a few more years. But hurrying just to show off to someone doesn't seem necessary to me at all. Being able to learn grade 7 pieces in reasonable time after 3 years seems adequate enough to me. I can spend a long time on this "level" without running out of wonderful music.
I will gladly post to the audition room as soon as I can make a recording I can feel satisfied about. But I really don't have enough time to securely memorize pieces, as soon as I can play them through there's new material to work on. My reading is really poor still... Some pieces are technically quite ready, but I still get memory issues so the recordings are ruined by little hesitations and pauses. Even bigger issue is that I haven't been able to solve the problems of my piano's sound on the recordings. It just sounds bad. Too loud for the room I guess and the things I've added for damping it down just make the bass sound horrible.
Would it make you feel better if I join your hypotheses that I would run into problems with advanced repertoire and would need to spend many more years on studying my knuckles? It makes no difference to me really if I say that you may be just right. Many things may happen in this world and I am no psychic. I just look at my hands when I play and make wild guesses...