Do I have to stop work if C9 EAD expires before receving physical green card as I-485 is approved?

Practically speaking, it would not appear to make much consequential difference as to what you do vis-à-vis your work where USCIS has already approved your I-485 adjustment of status application. If the unofficial online notice is correct, you should receive the official notice and green card shortly thereafter. The date should coincide with the unofficial online notice date. Technically speaking, however, employment authorization on the green card does not begin until you receive the green card itself. Your C9 card which I understand that you did not use is expiring next week, so you are authorized to work under that until next week. You have said that your OPT is expiring in 2022. So if you have not yet received the green card, you should technically be able to continue work on your OPT until you actually receive the physical green card. I do not believe that this is a fish or fowl situation. Interesting question. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.