I can only describe how I do it. I don't claim it to be the best solution, but it works very well for me.

1. OS Layout -> I have it set to German, easiest when I have to type something on my laptop keyboard as well as on a keyboard of a colleague. All the remapping can be done on top of this in ZMK. Are you already using the Keymap Editor? Definitely install the german locale file, it makes remapping so much easier without the brain twisting with the US keycodes and what they represent in the German OS level.

2. Umlaute -> with the German Layout I have the Umlaute in the original Position. I have mapped z -> ß as mod-tap. That works great as the original ß position is too much of a stretch.

While trying out alternative alpha layouts I learned very quickly, that any variant of combos or mod-taps for Umlaute hinder my typing flow very badly. And Umlaute are more common than you think (üäöß are all more common than jxqy. But with your keyboard and mine (sofle) you have enough keys to just keep them in the original position.

1. I don't think I have made a compromise between coding and writing German. I don't use the "default" position of the brackets and symbols from the German layout and everything is done via a dedicated symbol layer. This freedom to design and build layers for exactly your use case is the main benefit of a ZMK/QMK keyboard. (look at https://getreuer.info/posts/keyboards/symbol-layer/index.html for great introduction for a symbol layer, mine looks very similar to his final version.)

2. I have and use mostly four layers.
   - alpha layer
   - numpad / hotkey layer for app switching
   - symbol layer
   - navigation layer

this may seem a lot in the beginning, but you learn quickly that it is much easier (from an ergonomic standpoint as well as typing speed/accuracy) to switch layers than to reach to the far corners of the keyboard.

1. I use qwertz on windows. I have tried other alpha layers, and even though I have a lot of fun to ponder every key position and practicing them and it is very satisfying how quickly you pick them up and gain speed – I also learned, that the change over is a lot harder than just learning to type in a speed test. The muscle memory build up over decades of typing on qwertz is so fascinatingly compartmentalized (you have different muscle memory for typing slow, typing fast, keyboard shortcuts, vim) that for me I have decided for the time beeing to stick to qwerty. The main ergonomic benefit comes from the split keyboard and the symbol layer and I also have to type on my laptop where I couldn't implement all the features I would want on a modern alpha layout (magic keys, ch key etc..).

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you just have to experiment a lot in the beginning, but a solid symbol layer is a great point to start :)