Loading a custom library after all other libraries got loaded
That is neither possible (because JS files have no "weight" anymore in Drupal 8), nor does it make sense: Any module can declare a JS file as async or defered, so even if your file would be in last position in HMTL markup, there is absolutely no guarantee that it will be executed in that order.

PS: Module dependencies seem to quite reliably affect ordering of their JS libs in markup, not really certain about it though.