I'm glad I got most of my tweeking out of the way.
I thought those peeps had me cooked, until Emergo got me going.
I'm going to need all the time I can get to do the custom scenery since that's not my strong point.
Part of the answer lies in what Emergo and I experimented with. Another important piece is changing those stubborn peeps' minds. The last piece of the puzzle, hopefully you'll figure out before my park is released! 
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As said before: soooo curious to know what kind of tricks/pieces of the puzzle others applicated. For I must confess: I do have quite some "littlle tricks" by now.
But, like Todd said, "the smallest adjustments do seem to have the largest impacts"
Every time I test, I have to remodel/adjust other things to get it back to the former standards. This now becomes more and more difficult/time comsuming as more archy/buildings appear in my park.
A week ago or so, it seemed a near-perfect park: > 2000 happy peeps at the end of the 3rd year (rating 997!!! WOW), with my just 4 cells, and quite some peeps who made it through the whole circuit.
But then I added some scenery/buildings/shops and whatever, tried to keep the layout completely intact, but of course here and there a tile had changed and ....... testing again it seemed everything was dropped down the drain. Could fix it up again, but.......well, I will not finish this one in 5 (RL) years if I have to test and redo after every new shop/tile/building, will I ?
I'll go on though. Even if the scenery of this one is just at some 20%.
so, go on strong you all