From a purely logical point of view, when would it make sense to be controlling say three of the fans out of 18? Surely the logical grouping is either 'all 18’ or two banks or 9?
Can't be certain from the pic but I think only the Aquaero 6 came with a blue LEDs option, and that can take all 18 daisy-chained off one header by itself, let alone with Splittys. Were it me, I'd be more relaxed about whether it can handle it (it can) and decide based on the logical control groups you want to achieve and what makes wiring the simplest/neatest.
However - unlike how I did it - I'd suggest that you try to keep a standard tail on each fan. Partly to avoid the work in the first place but mainly because if you get a fan failure (and you're statistically more likely given your 18 to my 4!), you want to ve able to just drop in a replacement rather than dig out a soldering iron or pin crimper!