Hm, we are in similar situations; I started piano at 40 and have been at it for 21 years, though I didn't really make that much good progress until I found a good teacher 7 years ago. One thing I thing she would suggest about getting locked in to a way of playing based on muscle memory, which seems to be what you are describing is this. You have to discipline yourself to listen very carefully to the sounds you are making. That can be hard when you are playing something near the limit of your technical ability. So you can stick with a relatively difficult piece and record yourself and then make notes on the recording as you follow along with the score. Or you can take some time to play technically easier pieces. If you are playing that Brahms there are certainly lots of things within your technical range that are easier but still plenty interesting. Learn a couple of those easier things and then focus on playing them differently, different voicing, different articulations, different tempi. If the pieces are technically easy enough you should be able to break through the muscle memory of how you learned them and then be able to vary them musically.
Good options for this might be some of the easier movements from Bach's French Suites; there are not too many notes and yet there are all sorts of ways to imagine the phrasing, to bring out inner voices, to hear different lines as more or less prominent, etc. What you want to be able to do, for example, is here a bass line as, say, a cello part, with its own shape. So you listen to it carefully, imagine how you think it should sound and see if you cannot make it sound the way you want.
I think you want to get to a point where the thing that limits your ability to play beautifully is your ability to imagine how you want the piece to sound, rather than your ability to find the notes. To get there, I've found it helpful to work on less technically challenging pieces. I spent a whole year ditching my Schubert Impromptus and Beethoven sonata movements, just playing pieces from the easiest volume of "Music for Millions" as musically as I could; it was the most helpful year of practice I ever had. Since I've never actually heard you play, I could be way off base, but I hope it's helpful.