I actually “main” the CSJ in sompetitive play often so I could lend a little insight..
Personally I like to play to the strengths the weapon already has; it is very fast already, so add swim and run speed to taste. The weapon also builds special relatively quickly, so I don’t mind some Special Charge Up to expedite that either. The larger tank on the CSJ means a good couple of autobombs out of one tank...with Sub Saver or Sub Power Up, you have even more options to use these bombs in tandem with your main weapon. If you play a lot of turf war, Quick Respawn and Special Saver might be more your jam, keeping you alive and painting.
Important to note, the Custom Splattershot Jr. is actually a support weapon more often than not. It seems intuitive to paint the town and then slide up for point-blank kills, but the CSJ is often outmatched without the benefit of a flank. Instead, the CSJ shines when there is paint on the ground to surf around the map in, allowing slayers to push up knowing there is an autobomb/rain factory on their tails if things get hairy.
Concerning gear, a loadout that allows you to be mobile and continue to paint will prove to be beneficial. Here’s a link to a loadout that has worked very well for me:
https://loadout.ink/#000112298a2fd591640212dce
One suggestion that I might make is to replace the Quick Super Jump with Quick Respawn, as my build is kinda specific for the modes I play CSJ in.
Hope this helps!