Aren't you qualified for n400 by now? usually people get their i751 cases moved along w n400 from what i read, unless you dont want to become a citizen otherwise, I would call their service line to clarify with them bc they extended 18months the first time then another 24months but you said both letters only reflected extension from Oct 2020, in this case you could remind them politely your case is x months left to outside normal processing time, they normally would tell you to wait as there is still a few months but I think they will start review your case once you reminded them.
In the meantime, you could ask to set an appointment for I551 stamp for 1 year extension, if they say its too early to do that then ask to have your extension reflect to 42months [18+24 months, tell them you received the letters twice].
I agree w others that you wont lose your job just bc your gc is expired, in fact if it does happen, you can tell them you will risk losing your job in x month bc your case is still not yet being reviewed, then they will have to respond by giving you I551 extension or review your case.
However, as far as i understand this would not happen bc even if you change jobs when your i751 is not yet processed, you just need to show your new employer your SSN card + driver license to meet requirements on I-9 employment verification - thats it, nothing to do w your expired gc.
This happened to me after 3 months in the US, they had not sent me my gc, I called a zillion times to f/u w them for 3 months, they kept telling me to wait as they changed the gc production time to 120 days [in the past it used to be within 90 days and most people get it in 2-3 weeks w their SSN, in my case, I had nothing sent to me and had to f/u many many many many times ] upon entry during covid time, until the last time I said its less than 1 month of the production window, the agent over the phone still told me to wait bc there is still a few weeks left per gc production window, about 3 weeks later I got my card. So I guess on the surface, they would say they cant do anything, but on the back, for sure they are looking at your case - thats my guess.
This is not a professional advice though, look at your application details, do what is appropriate for you.