My suggestion would actually be to expand the control capabilities at this point. Instead of a regular Keystep, though, I'd suggest the Keystep Pro. The sequencer is on the order of complexity of the BSP, the keyboard controller feels decent, plus you get some extra controllers in its ribbons. But if the task here is to up the sequencing complexity, I think you'll find the KSP to be just the thing.
And when you expand the modular itself, then you'll already be set up to handle a more complex rig.