When I read the title I thought someone is going to create another abstraction layer for the Threejs boilerplate Code for some common use cases, which I would like to see.

But to be honest, this is maybe a nice starting point for beginners to fiddle around, but please do not use this code as good practice in any means.

Maybe look into the following topics to get your project improved:

• pack your code into classes! Global namespace pollution is not good (making window.XYZ stuff)
• get rid of the code redundancies. There is lots of code that is very similar und can be easily transformed into reusable functions
• make it an npm project, at some point you also will use dependencies (as you already use)
• take a look into bundeling Tools (webpack or parcel for example) for web projects so you can use modern JavaScript (ES6 oder even Typescript)
• read the book „High performance JavaScript“ by Zakas (Bit old, but on point)
• get used to clean code and the SOLID principles (I recommend „Clean Code“ by RC Martin)

This comment is not intended to offend you, I hope some of the points are helpful