There's nothing reasonable about your position. First and foremost, Y3120's TRMCA is set in Belarus during the early Zarubintsy (read Proto-Slavic) period, and forms 99% of all modern-day I2a1b-Din lineages in Europe, especially on the peninsula. It's highest diversity of clades is located in the triangle comprised of Poland, Belarus, and Ukraine - the most proposed landmass for the Proto-Slavic homeland. It's ABSENT in Tollense peoples, it's ABSENT in Gauls and other Celtic peoples (newest study from 2022 is free to read), and even among the closest East Germanic neighbors of the Slavs. One more thing, The I2a branch among Serbs, South Slavs, and Slavs, in general, is of the Din-CTS 10228, Y3120 (and all its downstream) variety, and wasn’t present in Southeastern Europe prior to the invasion of the Slavs. Slavs have also dispersed this Y-DNA to non-Slavic peoples, and the oldest historical samples of I2a1b-Din are found overwhelmingly in West and East Slavic lands, like KRA008 from Saxony, Niem6, and Niem13 in Poland (Stolarek et al. 2018; Zenczak et al. 2017), VK542 (Gleb Sviatoslavich) in Ukraine, and VK53 in Sweden (Margaryan et al. 2019), Sunghir6 in Russia (Sikora et al. 2017), KEII/16, KEII/52, KEIII/11, K2/16, K2/52, KR/12, and MH/9, MH/15, and MH/16 in Hungary, former Great Moravia(Neperáczki et al. 2019; Fóthi et al. 2019), and the Kuline twins (Olalde et al. 2021). However, we also have Early Slavic samples like POH36, POH41 and POH44, Byz3 (Lazaridis et al. 2022 - the Turkish flag beneath the Belarusian flag on YFULL, marked as ancient DNA) and Medieval South Slavic samples like UHR3, Knyaz Batinic-Mirkovic, and Knyaz Mirko Radojevic, (Kalajdžić et al. 2019), and the sample of King Stjepan Tomas from Bosnia and Herzegovina. This is the totality of I2a1b-Din's archaeogenetic samples, all are Early and Medieval Slavs, Western, Eastern, and Southern. This is the factual reality of Y3120's origin, and till they find pre-Slavic bearers of it, you cannot "postulate" a non-Slavic origin for it.
I find it bizarre that it's not just my people, the Greeks, who deny the obvious, and even empirically proven SLAVIC* origin of I2a1b-Din, but other Europeans as well. Just for everyone's information, I2a1b-Din is found strictly in a Slavic context, and outnumbers archaeogenetic R1a-M458 and R1a-Z282 findings, which unlike the I2a1b-Din, appear in preceding, non-Slavic contexts (La Tene findings, etc). To be entirely honest, it's not R1a that binds all Slavs, because there are different clades for different groups of Slavs, but I2a1b-Din, which is universally downstream of the Proto-Slavic Y3120 mutation. The Proto-Slavic haplogroups I2a1b-Din, R1a-M458 and R1a-Z282 expanded STRICTLY with the Slavs, that's it.