Nice setup for a case
I moved back to a case recently, and ended up doing the same thing, except with fans all over the place lol.
I like the homegrown power kit too.
Just FYI Look into a breadboard It is simpler and does not require soldering. they have been available for a long time, (at least since my first electronics kit in the late 70s) and will let you test out various configs before soldering.
the slots "hold" a wire in place for testing purposes so you can test out various serial/parallel configs at your leisure
EDIT: You will still need to tin your wires IIRC so not totally solder free, but easier than soldering desoldering
EDIT EDIT: If not sure you want single length or multiple lengths, just use sockets, and use extensions where necessary. You can get them prebraided wire covering cheap, like a buck or two per cable. Not as cheap as making your own, but.. still cheap
Think i paid 15 bucks for 6 from newegg at 3' length last time I needed them. Cheaper elsewhere but I needed them fast