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?
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.
Outside the League there's a spot on the right side of the path where you can take pictures of Vanillish and Absol. Absol gives anywhere from 9-12k points depending on framing if it's doing its pose and facing the camera when you take the picture. Vanillish is buggy and can end up stuck at max range facing away... but otherwise it gives between 5-6k when you take a good picture
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.
