I got it after 80 in-game days and constantly trying out what sticker works that involved dinosaurs / pride flags / race cars. Tbvh, I cannot guarantee whether or not even if you follow this guide, you'll get the achievement but regardless - I will still share what I did.

I started off making a sticker that involved all 3 elements (looked like ♥♥♥♥ honestly, I think I even deleted it mid game), then I tried making 1 each for each dinosaur, I tried making 5 individual pride flags of different kinds, and to finish it off - I made 1 for each race car and 1 sticker that literally only had 3 of the race cars and it prompt my achievement. All of the stickers were actively in my shop, none of them were de-listed.

I took some screenshots for reference, I do hope it'll work and hope the guide helps. Cheers!