generator transfer switch

Doing all the splicing in the load center is topologically equal to using the load center as a pass through. This is not permitted in some local cities' code.

With that many (six) circuits, the load center would be much cleaner if you withdraw the circuit cables in question from the load center and use a separate junction box.

Get a 59 cubic inch box for 12 gauge wiring (52 ci for all 14 gauge).

Be sure there are enough holes for the various cables going in. Usually one hole (with appropriate clamp) can admit two Romex cables.

Connect each circuit neutral only to the neutral that accompanies the corresponding hot wire going to a switch.

Connect all ground wires together in the junction box. You can group the ground wires into bundles with jumper wires (pigtails) connecting the bundles.