Like carve out a place between Russia, Belarus and Ukraine in the former Pale of Settlement. Establish a bunch of Yiddish language schools there. Have it become a center of Jewish culture. Sure, there are already Ukrainians, Russians and Belorussians there but there were already people in the far east too.
idk about Ukraine but the Jewish population in Germany was always pretty small, which is one of the key reasons they were able to do the holocaust.
Ukraine had a fairly large Jewish population before the holocaust. I think out of the 6 million Jews murdered during the shoa, 800,000 were Ukrainian, which is the second largest contingent after the Polish (2 million) IIRC.