Probably too late for this, but you can potentially cut the cost of your spacebar kit by nixing the 6u off-centered bar. The primary use case for that spacebar is a G80 1800 keyboard, which commonly requires 7x1u bottom row mods (1 Fn, 2 Ctrl, 2 Alt, 2 OS).
Never mind, just realized you have the necessary mods in the extension kit.