Nice river with beautiful surroundings, so jealous :-)
Music wise, I think you are going to give yourself a hard time to limit yourself on such a small case. Since you are good with wood, the additional costs would be limited if you would straight away make a 2 x 104 or 3 x 104 HP case, saves you later the trouble of extending it and you really don't have to fill it up fully, you can even leave two rows empty and just start with one row but at least you are prepared well for the (near) future.
Instead of two Doepfer A-111-6 modules you might want to consider just one and the space that the second would leave free use that for one or two more oscillator modules to get a bit more variation.
Maths is a very big module, sure it's nice but if you want to use it mainly for envelope purposes you might want to consider just one or two envelope modules that might be A) cheaper and B) perhaps a bit smaller in space.
I don't know the Unity Gain Mixer module, are you sure it's enough and sufficient to be used as an audio interface (input/output) module too? You might want to verify that, depending on your needs of course.
With your small rack you might not be able to get very complicated music, but then again, if I understand you right, you like to focus more on minimalistic music and for starters that might be a quite good beginning but as I said, please consider making directly a bigger case, that saves you near future extension issues :-)
Good luck with your nice youtube projects, keep up the nice videos and kind regards, Garfield Modular.