The Octatrack rabbit hole is pretty deep, lol. If you feel you might be in over your head, just focus on one thing to learn at a time. Set up a separate Project in the OT for each topic you want to explore.
I’ve noticed quite a few fellow OT users like to use other Elektron boxes as OT companions. Other OT “playmate” choices that seem to be popular: Korg Volca boxes, OP-1, iPad apps.
I just got a Microbrute myself. I already had a nice analog monosynth (Voyager), but for the low price, a simpler analog synth with no presets, one knob per function, etc. was just too appealing to me to resist. I’ll probably start off by just improvising noises with it and sampling it into Octatrack, then later dig into MIDI-syncing it with the OT and other stuff. However, I’m not sure the Microbrute is the best choice for a controller - its keyboard does not transmit velocity.
I like all the effects in the OT (esp. the filter, ring mod, and reverbs), but was missing a delay what would let me conveniently mess with time and feedback parameters, especially for getting feedback oscillation. So I got an analog delay pedal (Way Huge Echo Puss) that just happened to be on blowout sale.