You said it yourself. The reason why mine looks so good is that i can spend ages building on it, eventhough i really gotta say that my newer stuff is like 10 times better then my older stuff.
When i play planet zoo im in no rush, i just follow the best advise my art teacher ever gave me: Never do something what you dont feel like doing, do a bit here do a bit there and slowly but surly everything will be finished, nice and detailed and most importantly filled with effort and love in the details.
I just chillax and get done what i get done and when i feel like starting something else on the other side of the zoo then i do that.
This also means i often have things unfinished for a loooooooooong time, but when i get the moment of inspiration and start building it looks great in the end.
What also helps is that i got quite the collection of favored go to pieces. That otter bridge went really fast once i started it because its just my usual go to of arctic wood beams, africa low wood wall, mesh and more arctic wood. The fence for the water? Just bamboo and mesh. The other fence at the back and side of the habitat? African low wood wall + arctic wood decorations and the africa colorable painted beams.
By now i can make new custom fences that look great in under 10 minutes and its just some experience and experimentation.
Same for the plant pallete, as i mostly only use temperate, tundra, aquatic, grassland and taiga plants from europe and asia so by now i know how what i have and how to make them look good. It also meant that both europe and twillight were quite a treat regarding plants for me

Id also helps with overall zoo cohesion, if everywhere has roughly the same foliage and similar fences and pieces used, it looks more like they all fit together into one real place.