Will I be able to extend my Retirement Visa while stranded in the Philippines?

That depends NOT all immigration offices do the in country conversion from a 30 day visa exempt entry or a 60 day tourist visa entry to a 90 day Non-O visa <- the visa you need to get before you can apply for a yearly extension.

AND

For the offices that do the in country conversion you need between 15 and 21 days on your current entry stamp to start the process because the application goes under consideration for a period of time before you go back and get the Non-O visa and your new 90 day stamp inked in.

I'd say IF the borders don't open before you can get back on this current extension at least get a single entry tourist visa from a thai consulate where you are before you wing your way back (as that will give you plenty of time on your entry stamp to get thru the process.