Using Billboard outside a BillboardSet?

Is it possible to use Billboard without a BillboardSet and handle all Billboards manually?

According to the documentation it seems to require BillboardSet, but I’m not quite sure. At least it’s not possible to use Billboard as a component.

The reason I want to use Billboards without a set is because I want to have a dynamic sized set of Billboard and still be able to get the specific Billboard in the set by giving a predefined Index (such as a 3D array flatten as a 1D array).

Since a BillboardSet creates each Billboard beforehand it creates a lot of overhead memory wise.

You are tryin’ to do something unlogical. The main purpose of BillboardSet is to optimize things by grouping.
You would have x10 overhead if allocate billboards one by one e,g, in StaticModel. x100, if you create a lot of BillboardSets with single element.
If you want to have a lot of billboards without pre-allocating storage, you could resize BillboardSet on demand or create a few of them to avoid buffers re-allocation.