Hi Colo98,
Interesting idea of processing your acoustic piano but ehm... are you sure that modular will be the right solution? If yes, keep reading ;-)
As you indicated yourself, modular with just 84 HP is asking for trouble (space-wise). Go straight for 3 * 84 HP or 2 * 104 then at least you don't need straight away to buy yet another case.
I had a look at your rack but I can't find your audio (i.e. piano) input. Into which audio interface module does that go? I think you might have forgotten an audio input/output module? If you don't need a headphones output then Intellijel Audio Input/Output is a nice one.
Please carefully plan this rack since the limited space you have it will be very difficult to choose the correct modules for your need. On the other hand, once you got your rack, enjoy it :-)
Kind regards, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads