Here is an analysis of the modern Balkans. The ancient Balkans were genetically similar to ancient Greece. What differentiate each modern ethnic group is the amount of Steppe and Slavic ancestry that were added since the Late Antiquity. Unfortunately I do not have Early Slavic samples yet, so the closest proxies appear to be M****ithic Europeans and Bronze Age/Iron Age/Medieval Estonia. It will at least give a rough idea of the percentage of Slavic ancestry in each group.
Note that I prefer to use the 'Mycenaean Greek' and 'Italian Greek' as proxy for ancient Greece (Antiquity) rather than the 'Minoan Greek', as it is chronologically closer and already includes the Steppe ancestry from the Catacomb culture that came with the ProtoMycenaean invaders. For the Slavic ancestry, I give preference to Medieval Estonia, then IA Estonia, then BA Estonia and last to M****ithic Europeans.
Modern Albanians
3.84908822 21.60% West_Europeans_(WHG)_(n=19) + 78.40% Italian_Greeks_(n=2)
4.12423985 19.80% Balkans_HG_(n=43) + 80.20% Italian_Greeks_(n=2)
4.47182323 29.20% East_Europeans_(EHG)_(n=10) + 70.80% Minoan_Greece_(n=10)
4.57493695 21.80% East_Europeans_(EHG)_(n=10) + 78.20% Mycenaean_Greece_(n=4)
4.62179883 73.00% Italian_Greeks_(n=2) + 27.00% Swedish_Vikings_(n=23)
4.69283045 26.20% Unetice_culture_(n=18) + 73.80% Italian_Greeks_(n=2)
4.75311407 78.20% Italian_Greeks_(n=2) + 21.80% Medieval_Estonia_(n=7)
Albanians from Kosovo
3.60836736 27.60% West_Europeans_(WHG)_(n=19) + 72.40% Italian_Greeks_(n=2)
3.93050283 26.00% East_Europeans_(EHG)_(n=10) + 74.00% Mycenaean_Greece_(n=4)
4.19596010 33.00% East_Europeans_(EHG)_(n=10) + 67.00% Minoan_Greece_(n=10)
4.27918332 25.20% Balkans_HG_(n=43) + 74.80% Italian_Greeks_(n=2)
4.40733193 65.20% Italian_Greeks_(n=2) + 34.80% Swedish_Vikings_(n=23)
4.45984147 33.80% Unetice_culture_(n=18) + 66.20% Italian_Greeks_(n=2)
4.53617127 71.20% Mycenaean_Greece_(n=4) + 28.80% Bronze_Age_Estonia_(n=16)
4.55862432 63.80% Minoan_Greece_(n=10) + 36.20% Bronze_Age_Estonia_(n=16)
4.61941714 68.80% Mycenaean_Greece_(n=4) + 31.20% Iron_Age_Estonia_(n=8)
Modern North Macedonians
3.73357627 36.40% Balkans_HG_(n=43) + 63.60% Italian_Greeks_(n=2)
4.04875881 35.40% East_Europeans_(EHG)_(n=10) + 64.60% Mycenaean_Greece_(n=4)
4.30953564 39.20% West_Europeans_(WHG)_(n=19) + 60.80% Italian_Greeks_(n=2)
4.38159256 59.40% Italian_Greeks_(n=2) + 40.60% Medieval_Estonia_(n=7)
4.38999485 60.60% Mycenaean_Greece_(n=4) + 39.40% Bronze_Age_Estonia_(n=16)
4.47557289 57.40% Mycenaean_Greece_(n=4) + 42.60% Iron_Age_Estonia_(n=8)
Modern Bulgarians
Central Bulgarians
3.35929997 64.00% Italian_Greeks_(n=2) + 36.00% Iron_Age_Estonia_(n=8)
3.40257541 33.60% Balkans_HG_(n=43) + 66.40% Italian_Greeks_(n=2)
3.55530217 62.20% Italian_Greeks_(n=2) + 37.80% Medieval_Estonia_(n=7)
3.62855416 31.00% Baltic_Scandinavia_(SHG)_(n=31) + 69.00% Italian_Greeks_(n=2)
3.78559726 33.00% Bronze_Age_Estonia_(n=16) + 67.00% Italian_Greeks_(n=2)
Modern Montenegrins
3.36411366 54.60% Mycenaean_Greece_(n=4) + 45.40% Iron_Age_Estonia_(n=8)
3.40924239 58.20% Mycenaean_Greece_(n=4) + 41.80% Bronze_Age_Estonia_(n=16)
3.65528865 48.40% Minoan_Greece_(n=10) + 51.60% Iron_Age_Estonia_(n=8)
Modern Serbs
2.97481588 49.40% Mycenaean_Greece_(n=4) + 50.60% Iron_Age_Estonia_(n=8)
3.15613859 53.40% Mycenaean_Greece_(n=4) + 46.60% Bronze_Age_Estonia_(n=16)
3.40525200 43.80% Minoan_Greece_(n=10) + 56.20% Iron_Age_Estonia_(n=8)
Modern Bosnians
3.21062198 43.40% Mycenaean_Greece_(n=4) + 56.60% Iron_Age_Estonia_(n=8)
3.35892333 48.00% Mycenaean_Greece_(n=4) + 52.00% Bronze_Age_Estonia_(n=16)
3.48115419 50.20% Balkans_HG_(n=43) + 49.80% Italian_Greeks_(n=2)
3.50593787 38.60% Minoan_Greece_(n=10) + 61.40% Iron_Age_Estonia_(n=8)
3.73875455 43.00% Minoan_Greece_(n=10) + 57.00% Bronze_Age_Estonia_(n=16)
Modern Croats
2.96356284 42.60% Mycenaean_Greece_(n=4) + 57.40% Iron_Age_Estonia_(n=8)
3.42418040 47.20% Mycenaean_Greece_(n=4) + 52.80% Bronze_Age_Estonia_(n=16)
3.48083597 37.60% Minoan_Greece_(n=10) + 62.40% Iron_Age_Estonia_(n=8)
Modern Slovenes
3.17409255 52.00% Villanovans_(n=2) + 48.00% Iron_Age_Estonia_(n=8)
3.22750683 42.80% Bronze_Age_Estonia_(n=16) + 57.20% Villanovans_(n=2)
3.46088684 35.00% Mycenaean_Greece_(n=4) + 65.00% Medieval_Estonia_(n=7)
Here is a summary table. I also added Greeks from different regions.
Ethnic group 
Ancient Balkans (Illyrians/Greeks) 
Medieval Slavs 



Athens Greeks 
85% 
15% 
Central Greeks 
84% 
16% 
Peloponnese Greeks 
81% 
19% 
Albanians 
79% 
21% 
Thracian Greeks 
76% 
24% 
Thessalian Greeks 
75% 
25% 
Kosovars 
69% 
31% 
Macedonia Greeks 
67% 
33% 
Central Bulgarians 
64% 
36% 
North Macedonians 
58% 
42% 
Montenegrins 
55% 
45% 
Serbs 
50% 
50% 
Bosnians 
43% 
57% 
Croats 
42% 
58% 
Slovenes 
35% 
65% 