I think everyone should take the DSi and the Switch Lite as the standard for portability. I know that with portable PCs you can’t really shrink them much, but I don’t feel comfortable walking around with those huge devices on the street (maybe for travel), I prefer my portability to be small. I’m even thinking about picking up a PS Vita Slim, because it looks great despite the drifting issues and the worse screen. For phones I would only go back to playing on it if I buy a nice controller like the Elo Vagabond, I'm really picky with my controllers lol.
Discussion
As far as size GPD seems to get it right. There's a premium on their prices though!
https://www.youtube.com/watch?v=ewgSx-HtBHU
For the phones. I've heard good things about the Gamesir G8's. I haven't tried it myself, but I've gifted one to a friend who says he liked it.
https://www.androidcentral.com/accessories/gamesir-g8-plus-review
Wow, that really looks like a Vita 👀. Yeah, it’s a bit too pricey for me, maybe in the future. Gamesir makes great controllers, they have some really nice ones in their lineup