For those with an interest in ready made/commercial sequencers, Colin is on the launch pad with the successor to the legendary P3.
The P3 was originally offered as a kit/DIY, then became a commercial product, but proving, as with many other things like this, a rather expensive proposition to make.
He also does a well respected MIDI interface for 303’s, which allows, (at last!), MIDI control of the filter cutoff, as well as pattern download,and the usual MIDI control of the synth itself.
I have no commercial connection with Colin, apart from the fact that he kindly sold me one of his last sets of boards then I could build a P3, (which I couldn’t afford to buy). He’d probably be a bit horrified by my ‘temporary’ MDF front panel.