So what's the sense of it all? Perhaps too straightforward question (I know it's too general) but still - when there's the enlightenment why we even exists? If Samsara can be brutal and full of suffering, what's its purpose? To suffer and then enjoy how nice it is to not suffer? Or to have the choice whether to be good or bad? As humans we can surely limit the suffering. But how much really?

If there are 6 realms (hell, afflicted spirit, animal, human, asura, and heavenly beings) - what sort of principles could ever decide for such an order? Isn't everything supposed to be going towards the enlightenment? If that is so - what's the sense of having e.g. the hell? To "punish" those who act as evil so they could "taste" their own acting? I'm not really sure if I understand the purpose of going through all of it..

I believe it was Erwin Schroedinger who expressed an interesting idea that the total number of all minds in the universe is one. Just one mind. Split into different lives in different time and space. If that's the case I only wonder why would the same mind ever "start" this. Most likely it's only my limited point of view. Seems like never-ending fight between good and evil. Does the Buddhism somehow grasp such let's say - more (perhaps too much) general questions?
When there's the enlightenment why we even exists?

We exist believing in a permanent self because of craving.Enlightenment is when we let go of this craving and any other craving.

If Samsara can be brutal and full of suffering, what's its purpose?

I'll answer this question by asking another question.If you throw a ball up what is the purpose of the ball falling down?

There is none.Its just cause and effect.

Samsara was not created by a higher being and we were not put in it to learn some kind of life's lesson or as a kind of test.Remember,Samsara is not a place it's a state of mind of always craving.

To suffer and then enjoy how nice it is to not suffer? Or to have the
choice whether to be good or bad?

Enlightenment is when you transcend both.It's good that your disenchanted with this roller coaster of good then bad good then bad.Don't play the game anymore.If you want to be happy in samsara you'll need to accept the fact that you'll be unhappy when that happiness changes.Everything changes.Impermanence.That's about the only thing that's permanent with samsara.

As humans we can surely limit the suffering. But how much really?

Depends on our own effort.We can limit our suffering through our own merits but we will not be able to eradicate all suffering until enlightenment.

If there are 6 realms (hell, afflicted spirit, animal, human, asura, and heavenly beings) - what sort of principles could ever decide for such an order?

Karma.

If that is so - what's the sense of having e.g. the hell? To "punish" those who act as evil so they could "taste" their own acting? I'm not really sure if I understand the purpose of going through all of it..

We create our own heaven and hell.no one punishes or rewards us.Our actions creates these realms.To simplify things,If you don't want to go to hell do not act like you are already in hell.

Isn't everything supposed to be going towards the enlightenment?

could you tell me more about this.

I believe it was Erwin Schroedinger who expressed an interesting idea that the total number of all minds in the universe is one. Just one mind. Split into different lives in different time and space. If that's the case I only wonder why would the same mind ever "start" this.

I have heard of this concept in Gnostic and Hindu teachings.The closest thing i can relate to Buddhism is There is only awareness.But because of delusion we believe it's my awareness,my consciousness,my perceptions,my thoughts,my feelings,my body.My life.Infinite numbers of beings believing they have individual and separate egos.If i had to pin point what started this all i would say Delusion.But this is all beyond my scope.Please take it with a grain of salt.