I've been playing Planet Coaster since November and loving it, racked up over 200+ hours now.
I would say Spike, that when you start having large parks with 10k+ guests the struggling is caused mainly by the CPU rather than the graphics card. This game is one of the most CPU intensive games around and you really need a high end CPU to do the game justice. Sure, large amounts of RAM and a decent GPU is essential too, but the CPU is more important in this game, than say your average game, a lot of that is down to the complex and brilliant looking peep simulations going on. But I absolutely agree with you, the game is stunning in every way. The level of customisation and freedom you get is amazingly good.
I am running it on a AMD FX8350 8 core processor, 16GB RAM and a GTX960 and this game also struggles a lot on the huge parks with lots of guests. But in normal situations my PC runs the game pretty well. Hopefully going to get a GTX1060 at the end of the month so that I can run the game comfortably on the top graphics settings, that all depends on how much dosh I have left over after payday though.
I occasionally go on No Limits 2, mainly just to use some of the stunningly detailed rides people have put on the Steam workshop. No Limits 2 has just had a new update last week actually, completely reworking the lighting engine in the game, it looks stunning now to say the least.