Work on this project has been continued! I ordered a test PCB to try something new. Should arrive end of this week, I'll update my log then.
In a nutshell it'll be about getting everything thinner while making the whole unit bigger. I'll go for 100mm segment height and have worked on the "font" a bit. It's now easily scaled while maintaining enough space for the WS2813 LEDs.
New goal is to get down to 10mm thickness @ 100mm text height!
Talking about LEDs: I ordered 150x WS2813Bs and and 50x WS2812B mini (3535 case).