I think your calculations are off for the upcoming months. You said it yourself that the 880k per month figure comes from months without any bigger release, the best Switch had in a month before June this year was either one medium seller with Kirby or 2 remaster with Donkey Kong and Hyrule Warriors. January to May was a real drought for the Switch.
June is already shaping up to be a much better month for Nintendo due to getting lots of great games and not just one to keep the baseline going. July also has some great releases, though less than June because many of them were originally meant to release in July. August is a bit weak so far, and September is the month with the sports game releases, so that alone will push sales already.
What is however the biggest shortcoming of your calculation is that it does not acknowledge the boost in sales Switch will definitely have early next year after the 1-2 punch of Smash and Pokémon, plus whatever game may release in that time (maybe fire Emblem? Yoshi? Or something yet unknown? Time will tell).
So basically, you could increase the monthly rate higher than your 880k because it does not acknowledge what we already know about to come. At the same moment, Smash and Pokémon will lead to a, ehm, smashing holiday season with sales well above last year.
All that's missing is some promotional action like a pricecut or, more probable, a black Friday deal to fill up the gap to the 18-20M
I think Yoshi, Fire Emblem and Demon X Machina will push sales like Kirby did, so not a lot. I don't think the sports game in september wil have much of an impact on hardware sales on the long run.
I already added 120k per regular month in the calculation to acknowledge any hardware sales increase due to some games releasing like last week (YS, Crash and Wolfentein).
I agree I counted early 2019 like all the other months even if these months should sell more due to Smash and Pokemon.
So if we adjust the calculation i think it's ok to stick with 1'000'000 per month until early october and add a 33% increase for early 2019 (which is already quite optimistic i think) to 1'333'333/month.
Now it would be:
6 x 1'000'000: 6'000'000 (April to September 2018)
3 x 1'333'333: 4'000'000 (January to March 2019)
Total without holiday season: 10'000'000 Units
Leaving 10'000'000 for October/november/December, 3'333'333/month, including some bundles with smosh or Pokemon, or any black friday deals.
But I checked Wii's 2009 FY, it sold 20.7 millions:
Monthly sales from January to end of May : 1'180'000/month
Monthly sales from October to December: 3'888'000/month, so it's maybe something switch can repeat.