Yes I have said that Slavic languages are not older than 2000 years .
Till than people were speaking Balto-Slavic and around 2000 years ago,or later,Slavic split from Balto-Slavic and later Slavic expansion started,event which triggered the split of Slavic in 3 branches,South Slavic,Western Slavic,Eastern Slavic.
It is clear that the split was not that far away since South Slavic languages are still mutually intelligible.
As for the fact that Bulgarian migrated from today Russia to Balkans,that is shown by the strong mutual intelligibility between Russian and Bulgarian.
How Balto-Slavic speakers got on today land of Poland,Baltic countries,there is another story.
Thing is,Slavs got into Balkans around 1400-1500 years ago,not earlier than that.
The fact that they do not have borrowings from Latin language is showing that they were not in Balkans,in the period Roman Empire was present here.
Don't get me wrong but language and origin are two completely different things, and language is not prove of origin.
In the world there are a lot of natives in Asia, Africa, Latin America etc. whose language is English or Spanish but they have nothing with Europe nor with R1b, I1, etc haplogroups in Britain or Spain.
It is very logical, a tribe for thousands of years speaks own language. But in some time this tribe be conquered by invaders. For example and conqueror and tribe are same race.
If conqueror succeeded in imposing language and culture this tribe can change own native language in only two or three generations.
Imagine two or three thousand years passes and nowhere ex language is written.
Descendants can imagine that it is their long-time language and that they have same origin as conquerors.
However their origin can be completely different as evidenced by haplogroups.
I see you are Romanian. Romanians and Serbs speak different languages.
However, if you see Romanian and Serbian haplogroups you can see that Romanians and Serbs are very similar, you can compare percents different haplogroups in Eupedia (Eupedia is the best because Maciamo took data from all relevant studies in scientific Journals).
For example:
I2a
Serbs 33%, Romanians 26%
R1a
Serbs 16%, Romanians 17.5%
E-V13
Serbs 18%, Romanians 15%
I1
Serbs 8.5%, Romanians 4.5%
R1b
Serbs 8%, Romanians 12%
You can see for example Italian haplogroups by regions and see much more differences.
Of course, we can enter in dept and explorer further, but and now we can see similarity Serbs and Romanians.
But it should not surprising because Serbs are descended from Thracians, Illyrians, and even older settlers of the Balkans , etc. and Romanians are descedents from Dacians, Thracians, and even older settlers of the Balkans too.
Dacians were people similar with Thracians.
Yes, Serbs and Romanians have similar origin but Serbs and Romans speak different languages.
But Dacian language is not today's Romanian language, because Romanian language is adopted in process of Romanization during early centuries AD. Dacian language is not alive and there are very little written evidence.
But the origin is written in haplogroups, not in language.