<disclaimer>I have no mbSEQ nor do I know how it works, everything I say is mere speculation, also my comments go beyond the ui-level of design</disclaimer>
If the "fast" button is only toggles the speed of the encoder, you might wanna consider using an encoder with a built in button - turn to change the parameter slowly, push and turn to change the parameter fast. Otherwise you'd either have to hold the button with your right hand and turn the encoder with the left (the rest of the design makes it seem like you're right-handed) or hold the fast button with your pinky while turning the encoder.
The LEDs under the "step view button are way too close to it. There'll be virtually no panel between them. You typically want to keep the panel > 1.5mm in all places.
As ilmenator suggested - it's never too early to throw some real life measurements in there - the 1-16 buttons appear to be rather close to the display, depending on the body shape of the buttons.
Solo/Mute buttons: I'd probable go with different colors for the LEDs. Take a look at your favorite DAW - I am pretty sure they have different colors there.
Typically, it's incredibly helpful to print the whole thing 1:1, put some button caps and knobs on it and pretend to be using it. That's one of the easiest ways to find weak spots in my opinion.
HTH :flowers: