Thank you Lugia for your detailled answer, really appreciated!
About Reason : as said, I just want to focus on the hardware, so I'm seldom using Reason anymore. Anyway, during the rare times I was using it with the synths, I never noticed any timing issues yet.
The case : the Mantis case has 3A on the +12V distributed over 3 zones (1A each). So I guess there should be enough margin there.
Great tip on the input, I'll have a look at the other options to have more functions, could be useful indeed :-)
For the output : I've read so many different opinions on this, so couldn't get to a decent conclusion. Yours is the most motivated one I found ;-) So an attenuator like the Intellijel triatt followed by an output module (stereo) should do? I've updated the rack with it. There's so many options but I'd like to keep the prices not too high if possible.
Your essay was an interesting read. I plan on using the BSP to send the signals and already have 2 LFOs from my synths so it should get me started... slowly. I just want to build it progressively by starting from the basics. Along the way I might switch to other modules than the ones projected here, I don't know. I'm in no hurry at all as you tend to imply. The Clouds is here for now, we'll see if I like its replacement or not once it comes out, that can wait. This rack is not my first draft and I hit the 'del' button often! I was just to a point where I was satisfied with it and couldn't go any further without asking for opinions first. I'm using VCV rack to get used to the whole modular "concept" and see if it works for me. And Youtube has been a very good friend to discover new modules or check the ones I'd like to have.
Again, thanks for your answer. Cheers!