If you enjoy the workflow, the OP-Z cant really be beat when it comes to punch per square cm. But its one of those devices where you either love it despise it’s flaws or hate it because of them…
What it is though is an extremely powerful sequencer.
Polyend tracker is a solid choice as well!
Keep your Octa , its everything you need for this. If you want a synth go along side , get a digitone. If you want something quirky and fun , get the Monome Norns norns - docs
Considered a Deluge? It’s battery powered and does synths, sampling and sequencing. Haven’t used one myself but I have a friend who hated the OT and now loves his Deluge.
…most slimmed down set up i can imagine…
a mpc to host and sample whatever…
cause once u dive in, u’ll soon about to realise, this offers the only option to finish stuff for real, even without a computer…
a cycles to have also access to the other big hardware sequencing concept out there and for it’s truuly electronica vibe…
a mic to catch whatever u wanna catch…
a proper head phone to free ur mind, no matter what…
Ipad and a controller
My current answer is using the Digitone as mixer/sequencer/interface with max 2 devices: Obsession: what 2 devices to plug into Digitone L and R?
Digitone and a SP-404 is a fun little setup.
I think when the person arrives to buy the octatrack, you should immediately buy it back from them. It will be an early octatrack lesson for the buyer/seller 
That’s a wise decision. The Octatrack takes time to settle into. There is so many ways you can go with it that can cause indecision. Map out a plan on what you want from your OT and stick with it for a while. There’s plenty Elektronauts here that are more than willing to help you along the way.
yeah I think we need a better idea of what the OP’s current setup is to see what best to scale down to.