Hey,
I think you are really going to struggle to get much improvement if playing the game with a computer severely under the minimum specs. A quick Google says that this card is equivalent to a top spec graphics card from 2010-2012. Games get more and more powerful as the years go by and Planet Coaster 2 has a much higher minimum specification than the games listed that previously played ok, unfortunately.
I assume you have the graphics on the lowest setting already? Other options are perhaps subscription to GeForce Now which would enable you to play on a remote computer with the appropriate hardware.