It actually would be difficult unless you spent diamonds. I added up all the FP producing buildings in my city and in my inventory (a reasonable comparison to what others who don't purchase diamonds would have after ~3 years of playing) that provide an FP proficiency of .25/sq or higher (NOT including SOKs) and got this:
263 FPs/day @ 1266 sqs
^ Granted, the above factors in half the roads needed on the longest side of each building, so with some efficient planning, this could be reduced by about 40-50 sqs.
Add in your estimate of 370 FPs from GBs and that's only 633 FPs/day. Allowing some of those to get to lvl 100+ would make a big difference, but you specified no GB over lvl 81, so I'll respect that in my calculations.
According to the wiki there are 190 known expansions. If a city had all expansions earned and purchased with diamonds (big assumption here), then that equals 3040 sqs, so with the above considered, that leaves 1774 sqs left. But you have to account for the size of FP producing GBs, other GBs you also might have and want in your city that don't directly produce FPs (like CF), town hall, and any other needs for your city. Just town hall and FP GBs alone would take that down to ~1500. If you want CF, military GBs, etc.. your available space would again go down to about 1200-1300 sqs.
If you filled the rest of the space with SOKs, you'd need 250 of them and get 250 FPs, taking your total to 883 FPs/day. 250 SOKs wouldn't be impossible, but it'd be a royal pain, and I personally only have half that many. But assuming you did have that many...
Another factor is BG, you could technically earn over 100 FPs per day collecting from high output buildings like fully leveled settlement reward buildings and some of the more recent special buildings if the BG was leveled to 70+.
So with that setup you'd be pushing 1000, just about. But there's zero residential, production, goods, and cultural buildings and no room for deco. Of course if you used diamonds to purchase multiple copies of the more efficient FP producing buildings, it would be easier to reach that threshold. But even then, those buildings wouldn't save that much space, some of which might have to go to non-GB, non special event buildings to make sure you have the pop and/or happy to meet your needs, as well as any other buildings you might want or need. One trick to help work this out would be to keep all your FP producing special buildings that add pop in the lowest age possible and never use reno kits to upgrade them. That way you might meet your needs with only a high level Inno Tower, and since FPs in those buildings are the same across all ages, you don't have any reason to age them up.
Again though, without purchasing diamonds it would be very hard. Even with purchasing diamonds, you'd need to be very efficient and drastically limit the non-FP producing buildings in your city. Since most people prioritize atk % boost along with FP production, a pure FP producing city isn't very common to begin with.
Anyway, I hope all of that is accurate. I can get ahead of myself sometimes, so if I made a mistake in my calcs, feel free to let me know.