Should I optimize ground plane for AC or DC return currents?
Below 100KHz, currents explore all possible paths, proportional to conductance (actually its called susceptance for AC); consider some path that is 4" long; how does that path inductance compare to resistance of the copper foil (assume 1mm wide path).

The reactance of 100nH (4" wire) at 100KHz is = (2*pi*1e+5)*1e-7 = 0.0628 ohms. What is the path resistance (4" or 100mm by 1mm) with 100 squares of copper, at 500 microOhms per square? R = 50 milliOhms or 0.050 ohms.

Thus at 100KHz, the inductance of 4" path is approximately same as the resistance. The inductance is beginning to affect how the currents explore all possible RETURN paths.

Your SPI edges will be much faster than 100KHz, unless you slow them.