Given how long game development times are nowadays, Switch is bound to get seven years minimum before its successor launches. Given that it's now Nintendo's only console, eight years or more are also possible. When the 3DS could have six hardware SKUs, Switch should see at least five over its lifespan.
The DS had six years for itself (with year 6 still managing 20m units) and that will be the decisive difference in this race. Switch doesn't need to reach the same heights or maintain as high of a plateau as the DS because it will have more time.
Switch was at ~49m by the end of 2019, so a timeline like this would be sufficient:
End of 2020: ~75m
This example gives Switch a cushion of 10m to beat the DS and it's not especially optimistic. If Nintendo properly supports this console through 2023, then the above sales progression looks fine. 2024 onwards is when the year over year drops get sharper because the big hitters will go to Switch's successor. What the above sales curve assumes is a transitional period that gets handled similar to how Nintendo handled 3DS to Switch where the old system kept getting low profile games for a couple of years.
I think you've given a fairly realistic projection. Switch actually has a couple of major advantags that the DS didn't have at this point. The biggest advantage is the base model is still at launch price, and it's massively in short supply 3.5 years later. It doesn't even need a game bundled in with it. Switch has had one revision, the Lite model, but people are still willing to pay $300 (or more) for the launch model version of the Switch. There is still a huge amount of room for price cuts, bundles and revisions, and Switch doesn't need to do them any time soon. Even though I think this is definitely the peak year, annual sales can fall like a feather.
The other major advantage, as you said, is that they can afford to support the Switch for longer. Put all of this together, and Switch has very good odds against the DS.