Hi Juergen,
You seem pretty specific about what you want.
I would recommend the same thing as Kuba did. If your ZLF offers almost everything you need, then find a blade/woodworker and have him make a thicker rectangular handle using the same material from what yours is made from.
But as you noticed, when you change anything, the feel becomes different. A thicker handle means less of the impact sensation is transferred to your palm, it may also increase the reinforcement of the neck (where flex is most apparent). Have your chosen woodworker/blade maker experiment with different materials and the deepness and thickness of the "finger rest" portion of the handle (thinner and shorter to induce more neck flex).
I'm not the end all of blade EJs, but having owned 36 at one point or another, I like to think I'm moderately experienced. Going custom is like taking the "red pill". The more you try, the more you know. The more you know, the more discerning you become. The more discerning you become, the further you will feel like you are from perfection. What you think is perfect changes with the season. Every time you try a new blade, it challenges your existing thesis. And the cycle continues.
As your skills improve, optimal specs also change. But eventually you will reach a point where you can play competitively with just about anything.
I don't regret buying stuff because honestly TT is one of the (relatively) cheaper hobbies in the online community and it satisfies some on my curiosities. But you need to decide whether this bandwagon to nowhere is worth you while.