When I look at last year's summer, which had ARMS and Splatoon 2 as big titles, there were 651.377 Switch units sold between 4 June 2017 and 3 September 2017 (13 weeks, 50.106 units per week). And if we extend that to the end of September, we get a sales number of 857.525 units until 1 October 2017 (17 weeks, 50.442 units per week).
This Summer, we get Mario Tennis Aces in the ARMS slot - which I think will have higher software sales, but I don't think it will have much more influence on the hardware sales -, Octopath Traveler, Splatoon 2 Octo Expansion, and Taiko no Tatsujin in July. While those are all big hitters, I think those combined will also not have a bigger influence on Switch hardware sales than the initial Splatoon 2 release when that released at that point of the Switch's lifespan. Octopath Traveler may be the first JRPG of its kind on the Switch, but that's a genre that appeals to core gamers, who are more likely to be among first adopters and already have Xenoblade Chronicles 2, Atelier, Disgaea, and the promise of Fire Emblem.
I think the 50k/week line is a good one to follow until the first big September release, whether that's Smash or something else (it's not set in stone that Smash is a September title after all). That would mean the Switch would be at ~5,05 million units at that point. Maybe the actual number will be a bit higher, depending on the E3 promise Nintendo will deliver later today, but I think that won't be very significant to the total. I think your estimate of 5M to 5,1M is a good one.