Hatched over 550 eggs &

Keep hatching or hope for a lucky sighting.
It's one of the most rare mons, along with Unown.

Even from the Easter event? I wish I could tell you what type of stops I got Porygon Eggs from, but unlike 2 and 10k I always have 5k eggs in my egg inventory. They use to have random spawns near a train station.

I like near a major city and have seen some while driving through the city (as a passenger). Some of my friends also have caught porygons in this city too. The town still more immediate around me (rural) have yet to produce a porygon, from my experience

TL;DR: from my experience, a big city is more likely to spawn them compared to a rural area

It's really weird that you didn't see it during valentine's event. I think hatching is the best way.

Other than the Easter event, I found one at a Marina.

Once i saw once but I couldn't go I was busy... and then another I hatched. No more.

I've seen 88 caught 85 now, all in the same part of town which does not have a very high Pokestop density and Mon population otherwise. My theory is that they're attracted by this building https://commons.wikimedia.org/wiki/Category:New_Synagogue_in_Mainz , if I remember correctly they were all within 300 meters or so. Maybe look in industrial areas with similar structures