How make particles swarm around a certain area?

Yes, it’s possible. In MCreator 2026.1, you need to go to Mod Elements → Particle and create your particle. Then create a Procedure and set the trigger to "On particle tick update".
In this procedure, add the following blocks:

• Set particle motion X to (Get particle motion X + random -0.03 to 0.03)
• Set particle motion Y to (Get particle motion Y + random -0.02 to 0.05)
• Set particle motion Z to (Get particle motion Z + random -0.03 to 0.03)

Then go back to your particle and assign this procedure in "Particle tick event".
Result: the particle changes direction every tick instead of moving in a straight line.