Yeah, the problem is that in quick control mode, the last bank of knobs of your cc121 are assigned to your first four send levels, so you would loose those.
I agree that the quick controls thing is nice if you want to have specific parameters of various plugins availabe to control with ease of access, but the reality is that Steinberg’s solution is limited. They should have added emulation of MCU functionality to the CC121 and CMC series, so that for ex, you could push and hold a button, and you would enter an MCU emu mode where all the knobs are assigned to the parameters of plugins on insert slots, you push and hold another button and it would be the parameters of VSTi’s, you would push and hold another button, then the knobs are mapped to the cubase channel strip or at least the input section. As for your other request, you can assign a shortcut/command to the function keys.