I agree with everything in your comment except for your description of the Etruscans :-)
What I was trying to get at is that, by the time the Rig Veda was composed, the diaspora from the steppes was over a thousand years into the past. You wouldn't expect the composers of the Rig Veda to necessarily know anything about it to be able to mention it. Instead, you'd expect them to know even less about the migrations of their nation from the steppes than modern English-speakers know about Jarl Rikard. So the fact that the Rig Veda doesn't mention any long migrations is (almost) no evidence that the migrations didn't happen, nor that they were in any sense gradual. Especially since, unlike Homer, it barely mentions historical events at all—it's almost entirely supernatural bits.
What I was trying to get at is that, by the time the Rig Veda was composed, the diaspora from the steppes was over a thousand years into the past. You wouldn't expect the composers of the Rig Veda to necessarily know anything about it to be able to mention it. Instead, you'd expect them to know even less about the migrations of their nation from the steppes than modern English-speakers know about Jarl Rikard. So the fact that the Rig Veda doesn't mention any long migrations is (almost) no evidence that the migrations didn't happen, nor that they were in any sense gradual. Especially since, unlike Homer, it barely mentions historical events at all—it's almost entirely supernatural bits.