Installing access panel above electrical panel that is recessed into the stud spacing...

So, after some furthere investigating my OTR microwave situation, I really can't use my 15 amp option from the old range hood or jump off the 20 amp from the gas stove. Those circuits already have enough objects on them that it might cause a problem so I'm gonna run a new 20amp circuit, and after watching a few YouTube videos, I think it's something I can tackle.

Electrical Panel Side (Main Panel)
Black > Circuit Breaker
White > Neutral Bar
Ground > Neutral Bar

Outlet Side
Black > Brass Screws
White > Silver Screws
Ground > Green Screw

The "hard" part seems to be routing the Romex wiring through the attic.

Well, my panel is recessed into the drywall and there isn't any access above the panel to pop out the hole covers of the panel box and routing the cable through it. What I'm thinking of doing is cutting out a section of the drywall above the panel box and installing a spring loaded plastic panel door:




Some research has told me that if I were to do this, I would need a fire rated access panel if it's going on a firewall. My question is this:

What is a firewall? My electrical panel is in the garage, and on the other side leads to my backyard. So essentially, it's an "exterior" wall. The power meter is on this wall (outside of course). I should be fine with a plastic panel, right, or should I go with the fire rated option?