I'm not really sure, but you can travel back and forth between the city to other worlds to get fishbones (home, computer, fields, or woods, it's really up to you but apparently woods <-> computer is the best way to get fishbones quickly). This could help you level up your cats to be better.
Also I recommend checking out the Wholesome Cats Fandom Wiki. It has tons of info that could really help you, such as which acts can help you win which stages easily ;) Looking into the cats' stats is also helpful.
Personally for me one of the most important factors in deciding which cats to get is their type of attacks. I kinda classify them into 2 types: direct/close-up and projectiles. Projectiles is split-up into more types: slow/floaty and direct/WEEEEE. Direct are attack which requires the cat to touch their target to perform the attack. Projectiles, as long as its within the range, the cats can just stand there and throw. Otherwise, they move closer until within the range. The two types of projectiles are based on how the projectiles travel. Example: Leaf and bubbles go slowly kinda like waves; Flower and Cat just speeds directly to the target. I noticed the floaty kind usually misses pretty easily.
Other than that, I don't really know what to add. Sorry for the long text :') Must've painful to read for you.