Dibran, you have mixed branches. Branch L161 or so-called Isles has no expansion among Slavs. It is called Isles because many tested L161 people in I2a project come from the British Isles. It was found among ancient samples in Britain, but also in several places around Europe including Eastern Europe. Also. it is present in modern Europeans all around the continent in very small percentages. I do not know if there is a specific Slavic subclade of L161.
For Slavs important is parallel branch L621 which consist of Dinaric and Disles. Disles is very rare in Europe, almost non-noticeable. The name is given by a combination of Dinaric and Isles, but on the phylogenetic tree, it is closer to Dinaric or I-CTS10228. The supposed time of formation is 11200 ybp. with TMRCA 6500 ybp. Among modern Slavs there is the only expansion of I-CTS10228>>Y3120, supposed time of formation is 3.800 ybp with TMRCA 2200 ybp.
For prehistory and early history of young subclade Y3120č found mostly among modern Slavic population, the most important are early diverge branches of L621 that I mentioned above. Also very important is to find early diverged subclades of I-CTS10228. On the level of I-CTS10228 there are +25 known SNPs and on the level of Y3120 there are +6 known SNPs. Every SNP is a possible new branch, these branches might be present in modern populations of Europe or extinct. Before we found it in modern populations or ancient samples, we can only hypothesize about the origin of Y3120 before its expansion in Eastern Europe. We miss about 1500-2000 years on Yfull tree.
Problem is that many people take for granted theories spread on the internet that do not have any support in current results and researches from Population Genetics. Many times ****** spread it as ultimate proven truth. Unfortunately, there is no portal on the internet that summarizes information from public projects and scientific studies about L621 and other haplogroups.
Some theories you mentioned in your posts. One is that L621 took part in the division of Balto-Slavic languages. According to linguistic theories (which might not be true), the division between Baltic and Slavic language happened 3.500-3.300 ypb.
If there is a population/haplogroup that influenced that division, the TMRCA of that haplogroup might be at least 3.500 years, and bearers of that haplogroup had to have a significant number in that time.
Currently, we know that Slavic branches of I-Y3120 were formed 2.200 ybp. Yfull calculations are not perfect, we can feel free to push it a little back in the past e.g 2.500-3.000 years ago, but it hardly had a significant number of bearers in that time considering that it might survived a long time bottleneck.
For now, it is the most important to find early subclades, we know that some of them exist in the modern population of Upper Rhine basin, but it is not enough for the final conclusion.
About Bastarnae/Germanic origin it is useless to talk. Migration of Bastarnae is historically confirmed, but there are no early subclades in areal of early Germanic Cultures. As I mentioned those subclades are found in Upper Rhine basin. It might be some known historical migration but also prehistorical. The same thing about Celtic migration which might be more plausible.
About Y-STR diversity, there are not many papers about it.
Ken Notdvedt as an early researcher suggested that the highest diversity is on the of I-CTS10228 is on the borders of Ukraine, Belarus and Poland.
Dragan Primorac claimed the highest diversity in Dalmatia.
Olga Utevska, a Ukrainian scientist, in her dissertation suggested two regions, one is around Upper Tisza Basin, in general, North Carpathians (E. Slovakia, NE Hungary, W. Ukraine, SE Poland), other is Dnieper Basin. I shared maps and paper here.
According to public projects on FTDNA, the highest diversity of I-CTS10228 is around North Carpathians. Many subclades are confirmed there, but no any I-Y18331.
I do not know for any research that found the highest diversity in Moldova. There is only one paper with haplotypes without DYS448. Tested people were from NW Romania, Moldova and Ukrainians from Moldova.
There are 3 most common branches among modern Slavic populations I-S17250, I-Y4460 and I-Z17855. They have somewhat different dispersion and expansion rates. The most common is I-S17250 (about 60% of confirmed people and about half of them are I-PH908). The second one is I-Y4460 (about 40%), both branches can be found almost elsewhere, first is the most common in Balkan, West Slavs and Western Ukraine, while I-Y440 has high share in rest of Ukraine, Belarus and Russia, but there are members of this branch in Balkan, Poland, CZ, Slovakia, Hungary, Romania etc. The third one is I-Z17855 with about 5%, the branch is found mostly in Serbia, Bulgaria, Montenegro and region of Macedonia.
There is no expansion of I-Z17855 in Romania, Ukraine and Russia. Up to some degree, it might be present in Transylvania and North Carpathians. Most of the tested Romanians belong to S17250 and Y4460.
There are confirmed Z17855 in Poland, Slovakia, CZ, Ukraine and Russia, but its share in I-CTS10228 is very low.
Maps shared here are not accurate at all and no reference how shares were calculated. Making maps is not easy, first, you need to find all data from scientific studies to see the percentage of haplogroups than find tested people on deeper subclades on FTDNA or elsewhere. Looking at maps I can say only that guy who made it did not know the difference between Hungarian and Romanian surnames in Romania. Most tested Z17855 are Hungarians. But even if you know surname you cannot know ethnic origin of a person on the paternal line if it is not self-reported. Several people with Romanian surnames said that their paternal line is non-Romanian, administration in the past changed their surnames in Romanian.
Also about maps of Hungary and Serbia. In Hungarian sample are calculated people of South Slavic and Rusyn origin. On the map of Serbia are included dinaric north results of Serbs from Croatia and Bosnia with a higher share of S17250, while Serbia and Montengro have about 7-9% percent of I-Z17855 which is similar to Bulgaria and Macedonia.
Albanians in anonymous studies have about 9% I-CTS10228, about 3% are DYS448=19 and 6% percent are dinaric north. Inthat 6% are surely people negative on Z17855 because some of them might belong to Y4460, S17250 and even one haplotype seems to be A2512. In the time when maps were made only one Albanian was confirmed as I-Z17855>A1221. So how did someone calculate 9% of Z17855 in Albania?
In the best case it is 4%-5%? I wrote a lot about the possibility that most of Albanian I-CTS10228 are Z17855, it is the only hypothesis based on STR matches.
Vlach people are a mystery, several of them are I-CTS10228 (both dinaric south and north), no one is tested on deeper subclades.
Greece has the highest diversity of I-CTS10228 in Balkan for now, at least a half ethnic Greeks with Dinaric results are I-Y18331. How this subclade finish in Greece nobody knows. Except for Greece and downstream Jews, two people from Russia are confirmed positive on this SNP.
Bastarnae lived in Moldova and settled in nowadays Bulgaria and Dardania. No one is these areas is confirmed to belong to this branch yet. Also no historical sources about the settlement of Bastarnae in Greece, especially in the southern part of the country.
Slavs were bearers of I-CTS10228 for sure, but I would not exclude many other possible migrations, some of them are Carpi, Costoboci (that overrun Greece), Huns, Goths, Kutrigurs and so on. It not clear if some branches were present North of Danube and Carpathians before Slavs entered the area.