I started my Elektron journey with a Digitone, fell in love with the workflows and picked up a Digitakt as a follow-up. Once I confirmed (within a day) that the Digitakt was everything I had hoped for, I sold my MPC Live as being “too much” for what I wanted to do with hardware-based music creation. Picked up a Syntakt when it was released to “complete” the setup.
Me, I love the consistent workflows and complementary sound generation between these three units, but you’ll need to decide: do you want to keep with the Elektron Way, or do you want something completely different like the 404? And if Elektron, do you want the relatively easy transition to the Digitakt that literally has the exact same layout and almost identical sequencing as your Syntakt, or are you ready for the deeper challenge of an Octatrack?
Most importantly (from my perspective, YMMV), what are your specific musical/workflow goals and which sampling device best meets them in a way that is satisfyingly similar or different than the Syntakt and that you can grow with over time?
I personally LOVE the Digitakt with the Syntakt (and Digitone), but others also swear by the Octatrack/Syntakt pairing, which does seem beastly.
Just some points to ponder.