So I mean, most West/East Slavs are different to South Slavs. Wondering why that is?
How concretely they are different?
I have read that there was a tribe called the Sclavenians which conquered and ravaged the whole of the Balkans peninsula.
Sclavenians are mentioned in historical records but there were more tribes among them, same as Illyrians whose name covered all tribes in the Illyrian area.
Could they just have been dominant I2a?
If we take Croats as example, Slavic genetics of Croatians are R1a and I2a branches, possible others haplotypes as well, but this is the main one.
Croats speak Slavic Indo-European language but I2a peoples originally are not Indo-Europeans, for this reason I2a people had to learn Slavic language in the area where they were mixed with Slavs and Indo-Europeans. For now and based on living genetics I2a peoples are mixed with R1a peoples somewhere in the area of southern Poland and south-western Ukraine ie. that part of Carpathians.
For now, according to history records in that area White Croats are mentioned, possibly some other tribes but one of the earliest ones are White Croats.
Why is there this big genetic difference between north/south?
A lot of time has passed since migration period and normally some other genetics is mixed in the area of Carpatians, but in the Russians, Ukrainians, Belarusians little less in Poles etc.. there are still plenty peoples with I2a branches.
I mean I'm aware Slavs aren't a "race" rather a culture/language. But who were the I2a Slavs before Slavs existed?
They were not Slavs. If we follow this map original R1a Slavs has source somewhere in central Ukraina and from there they migrate to Poland and I2a peoples.
https://en.wikipedia.org/wiki/History_of_Proto-Slavic#/media/File:Balto-Slavic_lng.png
The same or similar talk and live genetics.
https://cache.eupedia.com/images/content/R1a_migration_map.jpg
Were they Dacians? The Balkan Slavs have many similar haplogroups to Romanians, moreso than to Russians and Poles.
Romanians has I2a branches with mutation I-Y3120 and I-S17250 which means that ancestors of I2a Croats and Romanians are same or have source in same area. Whether these(Romanian I2a) are some older or younger migration we'll know more in the future and then we will be able to say a little more.
I've also heard crazy theories about I2 coming from Celts/Gauls, this sounds wild to me, any credence to it?
As far as I know older I2a branches coming from direction of Luxembourg, northeastern France to Poland.
Or, if im understanding things about the haplogroup correctly, were the I2a Slavs just a minor tribe that had a population explosion and went south?
It could be because a good part of migration to the Balkan begins from a small area where I2a has its epicenter and it would be White Croatia, besides these Croats and others Slavs coming with their genetics and with regard to some other older I2a branches that exist in the Greece they probably go to the Balkans before Croats and via eastern Carpathians, through Romania, Bulgaria to the Greece. There are also Slavs from other areas that have some other R1a branches which says that they do not come from White Croatia but from Ukraine, northeastern Poland, Russia etc.