A fixed/fixed flip/flop is what my buddy did after the sorta-annual FG century ride turned into the now infamous "Death March".
He now runs a 48x17/18T for 75/70 GI, doesn't sound like much difference but the legs feel it.
You'll be limited by dropout length to how much cog difference between flip and flop so test thoroughly and carry a few wet wipes in your flats kit.

Time to decide on 1/2 x 1/8, track standard, or 1/2 x 3/32, road standard, chain.
I run 1/2 x 3/32 my buddy with big trackie power uses what he trusts, 1/2 x 1/8.
Quality machined steel cogs, not cheapo stamped, won't mess up hub threads, run true and last a long time.
Same w/ chains, quality matters when giving it all you have to get over that grade or spinning down it near hamster-in-wheel speed.

PS:
FG machines have a single chainring for good reasons, "leave the double on the crank" simply isn't feasible unless you plan on changing chain length mid ride to swap from 52 to 42 or whatever, which also poses chainline issues. Settle on a reasonable chainring size & rear cogs with the proper chainline and have it.

-Bandera