Exactly. As I understand it the invasions came as a result of a migration crisis, this migration crisis was also a symptom of the collapse, i.e. something else

Exactly. As I understand it the invasions came as a result of a migration crisis, this migration crisis was also a symptom of the collapse, i.e. something else (most likely devastating volcanic eruptions followed by nuclear winder, crop failures, etc.) which made life in Europe really hard, forcing the migration. I imagine these migrants weren’t super welcomed into the more rich Bronze age cultures, otherwise perhaps the migration might have been less violent, and collapse could have been avoided.