That is totally valid. Alternatively, using THREE.Sprite (which are technically Screen-Aligned Billboards).
Well, a low poly object has in general more polygons than a simple plane. Hence, a plane is often the lowest level of a LOD hierarchy. This is achieved by rendering the object to a render target and use it as a texture. The plane becomes to a so called Impostor.
You can directly use a SVG as a texture. You have to render it on a canvas and then us it as a data source for THREE.CanvasTexture.
It’s best to use JPG and PNG depending on whether you need alpha and can accept compression artifacts or not. For better performance, you can also consider to use compressed textures like Basis although it’s still a bit inconvenient to integrate compressed texture formats into 3D assets.