The difference between the Switch Lite and DS Lite is that the DS Lite was an objectively better version of the console to have. With the Switch Lite, you are losing a lot of the Switch's core features and limiting the amount of games you can play. Once things settle down I'm curious which SKU will sell better. I feel like in the US the regular Switch model will regularly outsell the Switch Lite, but maybe not in Japan.
Another difference is that the DS Lite essentially replaced the original DS, but the Switch Lite will continue to be sold alongside the OG Switch. It's like having two separate consoles being sold together, but both being counted as one console.