This looks like a really good starter system. There are a lot of opportunities to discover sounds by sending modulations from Pams to BIA and Sample Drum.
What do you feel like you're missing?
The 1u Midi interface would allow tighter integration with your Roland and Behringer
A dedicated audio mixer might be useful. There's an Intellijel 1u option or Mixup are designed to work with the case outputs
If you want to do manual rhythm sequencing Steppy or Varigate would be a nice addition. Take a look at Zularic or Numeric Repetitor if you want to discover interesting rhythmic patterns. Euclidian Circles does rhythms based on mathematic divisions
If you want melodies Varigate can be set for 2x Gate + 2x CV... and it can act as a master clock for Voltage Block. Some other good melody sequencing options would be: Scales, Pico SEQ, Super Sixteen or Bloom. People love or hate Bloom so research before you buy