Dang, my bad- I made a big fat error. The moment the mob dies, it ceases to exist- therefore the "execute at @e[tag=dummy]" portion would fail, because no entity tagged "dummy" would exist after you kill it. Yea, I think the best option would be to go with the passenger idea.

I think maybe you could modify this concept though, to spawn firework rockets, seeing as though the person uses it to spawn lightning bolts and summon armor stand corpses.

Detecting mob death as the mob is very limited as your cant directly execute on dead mobs.

You can detect if a mob dies from a player by making the killed mob have a passenger (such as a marker) before it is killed. When the player kills a mob, give them a custom player_killed_entity advancement and detect any of the passenger left behind when the mob is killed and execute a command on them

I know a command that could detect whether an entity is dead. Not sure how I would make sure the player themselves does the killing though.

/execute unless @e[​tag=dummy] run execute at @e[​tag=dummy] run ___________

I'm not sure how to spawn a firework using commands, I'll get back to you if I figure it out. The command to summon a firework goes in the blank space I put.

Also- you can give your mob any tag you'd like, so long as you change the "dummy" tag to reflect what tag you give the mob. You will need to spawn the mob using a command generator, and fill in the tag section with the tag you choose. Think of it as like the mob's name, but one the players can't see.