I've included a datapack that should give a fishing bobber visual fire after it is spawned. This is done with a custom enchantment that runs a function when a projectile is spawned.

Steps to reproduce

1. Install the datapack
2. Give yourself a fishing rod

3. /enchant @s minecraft:grill

4. Cast the rod and see that it isn't on fire

You can see that the enchantment does indeed work by trying it on a bow

/give @s bow[minecraft:enchantments={"minecraft:grill":1}]