If you don’t mind shelling out a bit of money I can vouch that GPU instancer on the Asset Store is amazing! I had 3,000,000 (Cubes) with colliders, etc and still had 70+ FPS and only a few draw calls (when i saw few I mean like somewhere around 10)… (Now whether it works on WebGL) I’m not sure, but if it does it - it’s literally the best money I’ve ever spent in my life on the asset store for anything. The performance gains are just (WoW).
Hope that can help you, at least as a last resort.
You can try getting in contact with the guy who made it
The asset store link is: https://assetstore.unity.com/packages/tools/utilities/gpu-instancer-117566