I know that the wither storm is not supposed to be killed in those phases but it still feels awkward having such a powerful boss like that drop nothing. A normal nether star or a command block could work.

Smite 5 netherite sword or power V bow with healing arrows. Also, I use strength 255 in creative for testing reasons.

It should probably drop a regular nether star but you're not really meant to kill it in the 0-3 phases

Yeah, it just feels awkward having such a powerful boss drop nothing.

How would you even kill it at the low phases considering it regenerates HP so fast?

Smite 5 netherite sword or power V bow with healing arrows. Also, I use strength 255 in creative for testing reasons.

Ah, now that makes sense. I did kill it once via Health potion with a modified level of 10 just to see what would happen.

Since those phases are technically supposed to be killed at all, we'll be leaving this suggestion out.