Which one of these 2 options is closer to your view? (or do you suggest a third option?)
I'd personally say a bit of both. There is a fundamental issue with you using the term "God" (capital G), which carries a specific set of definitions and assumptions (which form the basis of the part 2 here). Any given theist will (typically) belief in a specific god or gods, which will be entirely different to the god or gods believed in by other theists.
An atheist doesn't believe in
any god or gods. That will generally involve not believing in any of the specific proposed gods they're aware of and also not believing in the existence of any kind of god in general, however defined. The first statement will apply to the specific gods and the second statement would apply to the general concept.
1 If you go for option “1” you do have a burden proof, you are expected to provide an alternative explanation for the origin of the universe, fine tuning, morality, free will miracle claims and all the stuff comonly attributed to God, in the same way I can provide an alternative explanation for presents in the Christmas tree
I don't entirely agree. For a start, you're referring to the characteristics of a specific subset of gods (probably the specific god modern Christianity given the terminology). Not all god concepts are presented as explanations for all of those things, and certainly not in all cases.
But even beyond that, I don't agree that there is a
requirement to present any definitive alternative explanation to declare that a specific explanation isn't valid. If, for example, there are internal logical contradictions within the definition of a god or the explanation of how and why they cause these effects, we can say that specific god as defined can't exist. It is similar to someone accused of committing a crime but providing an alibi. If the alibi proves they were somewhere else at the time of the crime they can't have committed it (it would a be a logical contradiction for them to be in two places at the same time). They don't need to provide anything to explain who did commit the crime to demonstrate their innocence.
Anyway, "mundane" explanations for all of the things you list can be and have been presented. They're not always definitively proven explanations but they are at least as viable as the unsupported assertions attributed to gods and are therefore sufficient to support the non-belief in those
specifically defined gods
with the current evidence.
2 if you go for option 2, you have to give miracle claims a fair shake, you can’t dismiss them by default. You have to consider seriously the possibility of miracles. Or “god did it” answers.
Consider sure, but in the absence of any definitive evidence supporting a specific god as the definitive cause (as opposed to any different "supernatural" alternative, let alone any mundane ones), it will never progress beyond consideration. Ultimately, you can't use miracles to prove the existence of a god because you need to prove the god exists to present them as the cause of the miracles.
For example if we ever find the ruins of an ancient city on an other planet, you will naturally conclude that Aliens build that city (because “Aliens are not so unlikely)
Aliens as a general concept sure. Not any specifically defined type of alien.