Split digital power planes

Hello all, thanks in advance for your help!

I have a PCB I am creating that has multiple voltages, mostly 3.3V and 5V, with a few 1.22V pins.

My stackup looks like this:

Top Signal
GND
Inner 3 Signal

GND
PWR

Inner 6 Signal
GND
Bottom Signal


My thought is to have 5V occupy most of the PWR plane as those components are spread over most of the board. Then I would have a 3.3V island also on the PWR plane as all of those components are in the same vicinity, with a separation of about 20mils between the 5V and the 3.3V sections. Since there are only around ten 1.22V pins all in the center of a single BGA, I plan on having just thick (as thick as I can) traces or a small copper pour on Signal layer 6 to connect those to their regulator.

Now, there is a larger layer separation between the center planes and the adjacent signal layers. From top to bottom it is 6mil, 6mil, 11mil, 6mil, 11mil, 6mil, 6mil.

Is there anything I should do to the center GND plane, like splitting it along the same lines as the 3.3V and 5V power? Naturally many signals will be crossing that split, but those signals should be referencing either the top or bottom GND plane.

Obviously I've not dealt with split planes before, and I appreciate the advice of everyone here to help me get the best board I can.

Do not split the ground plane, one contiguous ground is better.
The signals on layer 3 are going to probably be coupled to that plane, and all the supplies are referenced to the plane, so splitting wont achieve anything positive.
If you can get hold of Henry Ott Electromagnetic compatability Engineering do so, chapter 3 is excellent. Also look for articles books etc by Howard Johnson (speeding Edge), Henry Ott and 'Right the First Time' Lee W. Ritchey. The latter use to be a free download, cant find the link at the moment.