If we rule out its predecessor to make things more interesting, which past Nintendo system does the Switch 2 feel the most like to you, and can you explain why?

None of them, honestly. It's a handheld. NES-Wii-U weren't. And the Gameboy-DS line were dedicated handhelds with their own games and own UIs etc. So the only thing the S2 can feel like is S1, only with a bigger screen and better specs. Sorry.

Like SNES, in that SNES as successor to NES kind of way, more of the same, but more powerful.

It feels more like the more technically capable Nintendo systems of their day simply because it's able to run "next-gen" third party games, something none of the Game Boy/DS/3DS could do and the Wii-Wii U could not do either.

So basically the comparable you have left are NES, SNES, N64, or GameCube.