Hi Luca, apologies for not responding sooner, (as one other member on the forum knows from messages - my time management at present is mad due to this V thing).
You sight read well, you know jazz style, you can play directly from a lead sheet with top line and chords - presumably you are then doing all your 2-5-1’s and tritone subs also automatically but (and this is a guess -) playing and memorising is completely different. … And hope that some method exists to aid you in the time it takes to to memorise a number… Well there are! Many.
Before going into detail I’m going to spell out - just so you know where I’m coming from - what ONE definition of how a memory system can help and why, and suggest one parameter that you can do a little experiment with.
I say this because there will be highly knowledgeable people lurking on this thread b who will consider what I’m going to say as “simplistic” or very far from the whole truth - and they’d be correct from their points of view. Never the less here it is:
In essence all You should try as a first step is to realise that memory systems in music are adding two things: you are doing everything else the same but adding (1) A DELIBERATE SPATIAL ASSOCIATION and (2) into that spatial part you are adding a SENSUAL ASSOCIATION of the information you are trying to memorise. The sensual part is (we tell beginners) visual representations, but really it can be anything sight, sound, taste, emotion, number/order, a symbol, a character you associate with some sensual association… it is a very long list… But I’ll stick with visual at present.
An experiment you can do - this minute - is pick up your fake book turn to random number - and instead of trying to play it, think of a specific place where you choose to associate this number with… I’ll give you a couple of example:. Dam Square in Holland or The Pantheon in Rome, choose just one. And from now on that number is associated with that place. What style you want to play it? Like Oscar? Well, now see Oscar sitting at the piano but studying the exact same page you are looking at. Imagine him in a specific place in your chosen location… There’s Oscar Peterson sitting looking at this for the first time… What would be say? What would he advise? This - seemingly mad idea is what after years of research and thousands of people working on this problem - is the STARTING point… You have a number, a location and one character. On this we build… But that’s the very first step…
After an hour doing something else, come back to the image in your mind’s eye and add more detail…
This is all that these methods are doing. But they can be incredibly powerful… But that’s how you start.
Try that, and tell us if that makes sense.
If it does, you can learn how to encode the chords, the harmonic rhythm, the top line, the modes and scales … all in the same step by step way… but it starts with spatial placement.
Hope that makes sense.