Nice tree!
Do you have a vision for the approximate 'final' height of this tree?
You mention the upper branches being too thick. In my view, those branches would never make it to the 'final' design. These branches can be, however, useful material to help with
development and it is a good thing that you have kept them!
1 - I think this tree would be a good candidate for the
development processs outlined in Meriggioli's
Bonsai Maples (see attached images). With Deshojo, it is appropriate to have a tall, slender, and gentle
taper, therefore my first
cut might be above the first, second, or possibly the third of the current branches. This obviously also gives you full control over the
movement in the trunk. Depending on the nebari, that first
prune might come anywhere from 1-5 years from now. (a master once half-jokingly said to me: "don't say 'chop', that's street lingo").
2 - Because the current trunkline does already have decent
movement, you could use sacrifice branches to introduce more
taper to the current trunk.
These are not 2 independent processes, and in most cases it is a combination of the two will get you where you want to go.
I think the biggest mistake one could make with material like this, would be to jump straight to
refinement.
Are you familiar with Brian's Chishio?:
https://www.bonsainut.com/threads/keep-it-or-cut-it.4383/
Brian's tree is in
refinement, not in
development like yours, but yours is not too far behind! Very promising material good luck!