I would just keep it simple. Hand wire a promicro or equivalent MCU. You could look at the BD9 from keeb.io for firmware inspiration (cough copy paste cough), since that is 7 switches plus 2 encoders. Each encoder has at least 3 functions: CW (example: vol up), CCW (ex. vol dn), and keypress (ex. play/pause). Total of 13 switches essentially, which is more or less what you need here.
For the lock switches, honestly not sure what would work best for this... You could program a downstroke macro and an upstroke macro. Depending on what media player you use, could be something like a low pass or hi pass filter preset or toggle an EQ mode of some kind, toggle input or output. A mute button would be simple too of course.
Lots of options with QMK, just need to put in the legwork. Hand wiring the encoder: I would be surprised if there isn't a tutorial somewhere online already for this.
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