Welcome @Fran_Acuna_Parra!
I believe you’re correct that 100k elements would be a bit much for BillboardCollection. It may be possible to improve BillboardCollection performance to this level — filing GitHub issues reproducing bad performance, or PRs improving performance, would be welcome!
These improvements are related to planned support for vector data in 3D Tiles and in CesiumJS. I know those GitHub issues seem quiet, but we are actively working on both, and you can expect more performance-focused options in the future. See the BufferPrimitiveCollection API proposal for some of the technical direction, though billboards and clustering are not discussed there yet.
Also, and available already — the new CesiumJS plugin by @vadymrostok announced at Plugin for massive markers rendering & animation - cesium-gpu-points-layer looks like it might be a good fit for your needs!