You are more limited by what you can do in the browser. If you are developing a mobile game not for the web and want the highest fedlity, you are best going native and using an engine such as Unity, Unreal etc.
If you are building a game for web, I would caution using engines that aren’t native as their web export is optimal (at the moment) compared to PlayCanvas, three.js etc .
For your actual problem:
I would look at LODing your complex meshes and downloading the right detail level at runtime pending on the device it’s been run on.
I believe a forum member has written a LOD plugin. Released: Hera LOD Manager