I made a resource pack to replace the particles from slimes jumping with slime block particles instead of slime ball particles, but I found that it didn't work as I expected and the particles weren't modified to the style specified by the resource pack, here are the contents of the specified file in models/item

{  
  "parent": "minecraft:item/generated",
  "textures": {    
  "layer0": "minecraft:item/slime_ball", 
  "particle": "minecraft:block/slime_block"  
  }
} 

I used the resource pack in the attachment and I found that the particle effect was not changed into slime blocks. (Issue.png)

After I removed "layer0", the particle successfully turned into a particle of a slime block, but some new issues appeared.

I used the same resource pack and when I tested it in version 1.21.3, the particles are particles of slime blocks instead of slime balls. (Expected.png)

Expected:

The "particle" field work when "layer0" and "particle" coexist in an item model file