The game has three monthly cards and I would recommend getting all of them to be honest. If you can only get 1 of them, Star Candy takes priority.
Like most gachas, the battle pass provides you with pulls and a lot of resources that you need to build your characters, worth getting the base battle pass.
The “premium” battle pass gives you 800 extra gems, decorations for your room and usually an animated frame. Up to you if it’s worth the increase in price, or you want to get the monthly cards instead.
For 1,000 gems you can get a random 3-star character of each element (5,000 gems in total).
It’s only worth getting these when the current banner is something you are not interested in. I would recommend getting all of them but the light one, since they have the weakest roster right now.
Skins are not limited in Trickcal so you can buy them anytime you want. They also include 100 extra Star Candy and room decorations, so this is the best way of supporting the developers while getting something you can really enjoy without gambling.