just a follow up to a comment I made a few weeks ago
One thing you could do for the cluster create command is make it have a specifier like this:
/witherstormmod cluster create (position and radius/pos 1-pos2) @e[type=witherstormmod:wither_storm,limit=1]

The specifier makes the cluster float and disappear at the specified entity, with it being able to go to any entity (just to make it a bit more simple, you can decide to add this part or not)

sounds better than having to manually edit the motion tag so it flies towards the wither storm

Will be implemented in the next 1.18./1.19 release