(Spoilers)... So, that ending....

We don't know exactly what happens to him after only it's implied that he kills himself.

A lot of the film is the grown up daughter looking back and like us searching for clues and hints from her faded memories of what it could have been. But the strobe scenes are supposed to be her kind of mish mash fantasy/dream/ of where he may have been and her trying to grab hold of him and stop him or save him but sadly he's always just out of reach and the visions of him scattered like a really old memory

Try to imagine someone from your past and its quite hard to 100 percent see them it's tiny flashes that you see so she is putting together the little flashes she remembers with her imagination of him after where she wasn't present as she desperately wants to know why and like we all would think i wish i could of grabbed hold of him and saved him, it's a very sad and cruel memory to have on loop in your mind all of the time.

It's up for debate if it was pre planned that was my thought but a lot of other posters think he was fighting it but succumbed sometime after Sophie went home
Makes rewatches of this film 'better' if you can say that because we as the viewer are very much like adult Sophie in looking for the signs and clues earlier on

I think, as the other poster said, you are taking it too literally.

The rave party seems something imaginary. It's where adult Sophie imagines seeing and saving his father.
Probably also related to how one of the last conversations they had was about drugs and parties.

I can imagine that having your father kill himself is a traumatizing feeling, that you will hold for the rest of your life, something that you'll always think "if I had known, I would do something to prevent this. What could I have done differently? Was part of it my fault?"