I generally agree with the comparison to the Gameboy series of hardware, though Nintendo should be wary of the Wii/Wii U transition, in which many appeared to believe the Wii U was simply an accessory to the Wii.
In my opinion, Nintendo should launch a Switch Pro that is fundamentally the same machine (i.e. uses compatible game cards, joycons, etc.) but with a higher-end feel (i.e. design, buttons), an improved screen (maybe simply the OLED from Switch OLED), and the ability to play games at a higher resolution/frame, especially when docked. Discontinue the OG Switch and have a tiered pricing system of Switch Pro (high-end), Switch OLED, and Switch Lite. Game cards generally can be shared (though the newer hardware will necessarily have some exclusives), local multiplayer works, etc. Create a situation where older gamers with disposable income upgrade to the latest and greatest, or maybe families can pass the OG Switch to the young sibling, and the older sibling gets the Switch Pro for his next birthday. Kinda sorta like the Gameboy back in the day.
If this is the plan, given the current Switch lineup's ongoing momentum, it makes sense that Nintendo would not announce a new model until after this holiday season. And, yeah, I think a $399 (Pro), $279-$299 (OLED), and $199 (Lite) is feasible.