Well first problem is Unity will eat lots of ram from devices, there is no way to fix this, all Unity games suffer from it thus the bad reputation of unity comes from it. Next is your models are way to high definition , you dont need the best models, you need the visuals. My recommendation is ditch Unity if you want to upload this on HTML with the 3D models and use https://www.babylonjs.com/
I would also made the models have less polygons so people wont have to wait 5 mins for it to load.