2020's focus also, hate to say it in the gaming world will be the PS5/XB2 launches, they're the new systems, they will get the lion's share of attention. Particularly the PS5 as it is the successor to the market leading platform.
Historically, the release of a console in one brand has absolutely zero impact on sales of a console from another brand. The console market is not a purely zero-sum game, because the sale of a console from one brand does not necessarily entail a lost sale for the competitor. There is overlap. This is especially pertinent in Nintendo's case as they've been making distinctly unconventional consoles since the Wii, so it's not like they're providing the same kind of experiences PS & Xbox offer. Remember the report saying that 70% of Switch owners also owned a PS4 and/or XBO? Even with the Wii, the available evidence suggests that at least half of all Wii owners also had a PS3 and/or 360. People who play on conventional consoles by and large haven't forgotten about Nintendo, and many of them actually want a Nintendo system to complement their PS or Xbox. I doubt that will change come Nov. 2020. The only thing the PS5 and Scarlett will affect are PS4 and XBO sales, as the post-peak decline in sales of a console often accelerate once its successor is released.
As to whether this year or next year is the Switch's peak, I think that will depend on if and when new models and/or a price cut are issued, and if those produce an equal or larger boost to sales than what Pokemon Sword & Shield do this November.