Ilmen Slavs - called "Slovenes" or "Slovenians" or "Vens" in some sources

Those people didn't call themselves that though, it's a later academic term. They called themselves "Slovene", it's plural, singular is "a Sloven" (male), "a Slovenka (female)". Ilmen' or Ilmer' is the lake near which the city of Novgorod was built. Slovene migrated to that Baltic area by the sea from Pommerania, Wagria and Rugen in modern Germany, which back then was Slavic. I guess their best English rendering would "Slovene" (slo-VEH-neh), so that they are not confused with modern Slovenians, which back in those times were called "Horutane" (Carantanians).