My DEROS is January of 2021 and my ETS is Sep 21. I want to retire at my current duty station so, would I have to submit an FSTC to extend?

I’ve been in 10 years so far. And yes, I want to ETS from my current duty station.SSG Laurie Mullen

SGT (Join to see) - Ok, then you won't retire, you'll just ETS. There is a BIG difference between the two.

Oh ok. So, would I have to extend to adjust my DEROS to the right to prevent falling on orders?SSG Laurie Mullen

SGT (Join to see) retiring is not ETSing, big difference.
You will be placed on AI at 7 months prior to DEROS. If you are on a short tour it’s a 6 month SRR. If you are on a long tour it’s a 12 month SRR to return to the states. If you have to extend to meet the SRR you can decline to reenlist and your DEROS will be 2 days prior to your ETS. If you don’t need to extend to meet the SRR then you will PCS. Whether you are allowed to FSTE is up to your branch manager.
It’s FSTE not FSTC. Foreign Service Tour Extension