I wouldn't change to SSD just for a game, the speed difference would be barely noticeable. Where you do notice SSD speed is with the time it takes Windows to boot up or heavy programs like PhotoShop to run. You wouldn't notice it in a game.
I run 32GB of DDR4 ram myself but I suspect it's overkill6GB on a video card is more than plenty.
I thought 8GB of VRAM was plenty. Until I played FF XV.
SSD usage really depends on the game. Some games (like cities skylines), benefit a lot from initial loading time, to ingame streaming textures while playing.
But it may look like Planet Zoo will load everything from the start. And without a PZ's version of thememakers toolkit yet. So installing the game in an HDD should be fine for now.
Always leave your HDD and SSD 20% free.