I actually make fewer errors and in some cases reach higher speeds when typing a lot of special symbols as everything is reachable without moving my hands and I arranged all special symbols in a way that makes sense to me. Muscle memory works perfectly fine, with the difference that I don't need to make blind error prone hand movements across the keyboard to use arrow keys etc.

That sounds promising. Training muscle memory for a custom arrangement of keys is something I'm not (yet) brave enough to do. Did you arrange everything yourself?

Normal Arrow-Keys on a layer are good. But I need Arrow-Keys usually in combination with Shift and Ctrl. (move to start, end, next instance, declaration, implementation, Select something). This is already overloaded a lot, so I'm not sure, how well this works with a layer.

The thing that's missing for me most are those function keys. I need them a lot - sometimes overloaded in combination with Shift and Ctrl. Those actions are usually something that should be out of reach from normal typing (Terminate something, start a Compiler, mess with Breakpoints, ...).

Those small keyboards never include function keys. But I think I could just build something for that.. Custom button-Panels with a few keys are quite easy to make.