when you say Octo boards with an S, it's a bit scary

you can use splitters to connect all your fans. one octo can manage a ridiculous number of fans already. If you have two 480 rads and say a back exhaust fan, you can use one channel per radiator and one for the back fan.
For the connexions, all should be explained in the manuals.
It's a long list, but there are english manuals to download :
https://aquacomputer.de/handbuecher.html
Octo, D5 Next and Leakshield connect through USB to the motherboard. there's no interconnexion between them. it is all done through USB.
for the D5, sata power to the power supply, and USB to the motherboard USB2 header
For the leakshield : usb connextion to the motherboard
For the Octo : PATA (molex) power from the PSU and USB to the motherboard.
The D5 is controlled as its own device on Aquasuite, you don't control it with the Octo. the Octo will only serve as control for your fans.