Yep, that's a lot of inputs. Definitely seeing the appeal of a nice big 10ch mixer!
For feedback on the current proposal though, I'm with kolderman on using a switcher per input type. Personally I would set up like this:
- 1x 6ch switcher for the MIDI devices
- 1x 6ch switcher for the sound cards + SCART switch
- These feed a cheap little mixer like a Xenyx 502 (taking up both available stereo inputs)
This leaves you a couple of spare inputs on the MIDI switcher. You never know when you're going to add an MT-32 or FB-01 😉 Technically you'd still have a spare input on the Xenyx as well; the dual RCA CD/tape input. It runs straight through at line level and doesn't have its own channel controls, but can come in handy.