Need help designing a circuit layout and wiring diagram for a garage
Breaker

Be certain you are using a double-pole 20A or two handle-tied 20A breakers for your 12/3 multiwire circuit.

Share the load, Frodo!

What I have drawn can essentially share your potential loads equally between your two branches if you wire half the garage receptacles to the LOAD side of one GFCI and the other half to the LOAD side of the second GFCI. I would focus on splitting up the two workbench receptacles between the two branches since they're likely to see the most load with power tools, etc.

Lighting

You can opt out of splitting your lighting if you'd like because your lighting load should be minimal. I drew it as split just for further example.

I also drew both light switches in a single junction box. This obviously will be determined more by where you want to physically locate the switches. However, the two gang box works well to handle the wire and wirenut fill that I drew. Again, this can be distributed depending on where you locate your switches.

Diagram

It's no work of art, but I hope I made it understandable.


Neutrals are dashed lines.
Grounds are implied. (don't forget them!)