I also usually add extra instance related functions to instance.js.
However, if you have a plugin which shares information / functionality between instances, I will also add properties and functions to type.js.
I have also created separate '3rd party' files which contain classes that I use in a project (for example in the Spine project, I have a class that batches drawing all the instances. The class also contains a list of all instances.) If you do a 3rd party file post r226, watch out for the recent module globaThis/import/export changes.