EMI concern about the length of return traces
1 MHz is fairly tame, not much above audio from the perspective of EMI. For example it is well below the self-resonance of most discrete components, and its quarter-wavelength on a PCB trace is around 4.5 m.
My instinct is to slug the FETs to limit their output bandwidth to around 5-10 MHz. That looks a little lower than the LEDs' maximum bandwidth, which will also be a kindness to them. Decoupling capacitors (usually ceramics in my day) should do the job.
Otherwise, provided you run the traces sensibly, I can't see EMI being a significant issue.