So basically I2a Slavs were probably living with R1a Slavs, sharing the culture and inter-mixing? Then during the Migrations Era most of the tribes heading south were I2a dominant, replacing whatever the Illyrians/Thracians were (possibly dominant E1b)?

Just saying, lots of us South Slavs look much different from Poles and Russians. It makes simple sense that we're mostly locals, I mean did the Illyrians and Thracians vanish? However, I must say too, whatever the case is, we're definitely mixed. I took a genetics test which says I'm I2-L621 on my dads side and H11 on my moms (H11 being common in central/Northern Europe but originating most likely from Russia).