The level 5 upgrade for the Poké Finder requires 1,500,000 thumbs up. I have almost 150,000 total thumbs up, and I can't imagine getting that many with just 2,000-3,000 for each photo.
Where are the places that offer opportunities for photos that will get the most thumbs up?
Best Answer
In this thread where a user announces what the V5 upgrade is (spoiler alert: it doesn't seem worth it), someone mentions a place that they personally used to grind to V5 and other users have said that it helped them a lot.
I can't find any other locations that would give this many thumbs up consistently, so it seems to be your best bet.
Naturally, getting photographs of the "rare" Pokémon in an area will get you the most possible thumbs up (someone in the above thread said that managing to snap a Dragonite got them 37k thumbs up), but it may take you longer if you rely on finding rares, as you'd be relying on chance.