Since the preorders ain't up yet, I just expect a moderate loss of speed for now. However, once you can order the Switch Lite, sales should tank for a while... and then go through the roof once the Lite is out.
I expect the lead to shrink only to about 200-250k until the Lite comes out, but regrow very quickly afterwards
That's quite the drop you expect, an average of 10-15k units per week under last year's sales over the course of the next ten weeks.
I am not even sure if the current lead of ~350k units will be lower in ten weeks time because Japan's preference for handheld consoles tends to be overstated. The original Switch may cost more, but it also doubles as home console, so it will remain appealing. Fire Emblem: Three Houses is coming up, so weekly sales should remain above last year's weekly sales or at least hover right around last year's weekly sales in the coming next ten weeks.