Hi Antti,
Oh you are already the third or fourth person in a row that got an Octatrack and you also got yourself a 0-coast, I am so jealous :-) So that with an Eurorack should go nicely together!
Regarding your rack, well you got already a 0-coast and an Octatrack, so that’s a good start. A few matters you might want to look into it:
I noticed the Veils (Quad VCA), so that’s good however I am missing an audio input/output interface. How are going to output your Eurorack sound? Everything via the 0-coast to the Octatrack? That might be a bit limiting. I think you should consider an audio input/output module for your Eurorack so you can lead the sound directly from/to the Eurorack to/from your Octatrack
Since you mentioned yourself already that you might want to extend in the future, perhaps you should reconsider your choice of mixer, the A-138s. If you take the A-138p and A-138o, it’s indeed a bit bigger but ready for the future, it has panning & aux options and the A-138p is chainable with more A-138p’s so you can just simply start with one A-138p and the A-138o, gives you 4 channels and once you require more channels you just add an A-138p to get to 8 channels (or 12 or 16, etcetera). I guess now you want to do that all with your Octatrack, understandable however if your Eurorack gets bigger you might need a bit more mixing possibilities before you go out with your Eurorack sound to your outside world (i.e. Octatrack)
Other than that it looks quite complete to me, you mention yourself already that effects, etcetera you want to do with your current external devices (for example the Octatrack), so that saves indeed some space in your rack.
Perhaps you could consider one or two more oscillators and/or filters but you also can keep that in mind for the future and just start with what you got here. Together with the 0-coast and Octatrack I think you find yourself here with some nice stuff to play with :-)
Good luck with the Eurorack and kind regards, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads