Circle of the Swarm

Druids of the Circle of the Swarm specialize in not turning into just one animal, but multitudes. Able to envelop, surround, and overwhelm foes entirely on their own by splitting their consciousness across many bodies.

Swarming Hoard

When you choose this subclass at level 2, you gain the ability to assume the shape of a swarm of animals. When selecting which shape to assume when using your wildshape ability, you can choose a creature with the Beast type that has “Swarm of…” in its name. When assuming the form of a Swarm, you may choose a creature with a Flying or Swimming speed regardless of your level, but must still obey the restrictions on the creatures CR.

Divded Form

Also at a level two, you learn to divide your Swarm body into its individual parts. While you have assumed the form of a Swarm, you may use a Bonus action to divide your body. If the form you’ve chosen has a statblock representing an individual creature that makes up your Swarm (I.e. a Swarm of Rats is made up of multiple Rats; a Swarm of Quippers made up of many Quippers), you may split your body into a number of individual creatures equal to 3 x your Proficiency Bonus.

You control all these individual forms simultaneously, and receive information from each individual form at the same time. In combat, these forms share the same initiative and take their turns one after the other. If an individual body is reduced to 0 hit points while at least one other individual body remains, it is destroyed. When the last individual remaining is reduced to 0 hit points, you revert back to your normal form, and any remaining damage is carried over to your current HP.

When divided in this matter, you can use a bonus action to return to your Swarm shape if at least half of the individuals are within 5 feet of each other. Furthermore, while divided, when you use an action to revert back to your humanoid form, you choose an individual whose place which you wish to occupy. When reverting to your humanoid form this way, all other individuals disappear into thin air, and you take one point of Force damage for each individual not within 30 feet of you when you revert.

DNDbeyond's a buggy mess and requires me to put something at 3rd level if I want to publish a druid subclass

As the title says

Conquering Form

Starting at 6th level, your swarm acts as a cohesive unit even when split apart. While in your Divided Form, the Independent creatures you control grant advantage on all attack rolls made by an ally or another one of your Independent bodies within 5 feet of it.

In addition, while in the form of a Swarm or while you're controlling Independent creatures through your Divided Form ability, you may use your Wisdom score when determining attack and damage bonuses.

Bolstered Mass

Starting at 10th, when assuming the form of a Swarm, add an additional hit die to its health points equal to your half Druid level, rounded up. This bonus applies to each individual creature when you first assume a Divided Form during your Wildshape.

Animal Kingdom

Starting at 14th level, when you use your Wildshape to turn into a Beast without “Swarm of…” in its name, you may create and control a number of individual creatures equal to 2 x your Proficiency Bonus , controlling and treating them as if they had been created with your Divided Form ability.