I would think that if you connect both the digitakt and minilab via USB then the midi between the two is connected and the minilab CC's also go straight out to the other ports on the RK008. But perhaps it is also possible you plug in the minilab straight in the digitakt (if the minilab has midi out as well, not only usb) the midi from the minilab should be forwarded by the digitakt (and going into the RK006 will be forwarded to other devices too)
But I'd also say: don't thru the digitone, typhon, lemondrop like in the schematic - the RK006 is made with a bunch of ports and if you make use of that you'll eliminate the latency from the thru-scenario you have drawn out.