It's not quite a modern smartphone, but it's the best we have at the moment; have you looked into the PinePhone? I have mine running openSUSE Tumbleweed.
I love my Pinephone. It is undoubtedly my own, with no strings or trillion-dollar corporation helping steer. It's lots of fun to play with, but unless people already half-jokingly compare you to RMS due to your extremism, it's not ready.
Android circa 2009 would be a reasonable comparison: the potential is clear, the software is rapidly evolving, and there's a benevolent dictator at the helm. And that's enough for me to be happy with it :)
The basics are pretty much down. Kernel support is solid. It can make calls, send texts (MMS mileage may vary), and use data pretty reliably. Web browsing is actually pretty fast with Angelfish. You technically have access to the full repository of Linux ARM software, and some of it even resizes properly to the phone. The camera is usable but terrible. Anbox works for Android apps but is painfully slow and can't share data with the rest of the phone to my knowledge.
Battery life is terrible, I don't think that the phone has power states of any kind, so it's either with the screen on, on with the screen off, or off altogether. Updates frequently break my install, although updating through SSH has been working for me recently on Tumbleweed without breaking anything. Little things like Plasma not having a way to exit the keyboard, apps taking up full screen with no way to exit them, etc.
Performance is painfully slow, but has also improved (for KDE anyways) by leaps and bounds. It used to be completely unusable but now it's merely very slow.
I would say it's somewhere between for developers, and usable, at this point. You could use it with some sacrifices, and still have a functional wireless communication device. It absolutely is nowhere near replacing my OnePlus running Android, however.
Sure, not quite as handy — or Handy, for the German-speakers among us — as a physical phone... But, say you keep your laptop with you in a backpack (Rucksack ;-) ) or such, and a Bluetooth hands-free headset clipped to your ear...? I hear lots of youngsters listen to music continually nowadays, so they already have some kind of earbuds in all the time anyway. Or maybe even some kind of Bluetooth "satellite" handset, to make it easier to initiate outgoing calls / read and write text messages?
Money: Pine64 is a small operation with limited resources, factories have minimum order quantities among other commitments.
Most Pine64 products have pre-alpha software and are aimed at volunteers who can improve it. Lots of people are willing to buy a product for <$150 and "see how it goes". $400 filters out a lot of people who might otherwise chip-away at software bugs on weekends. Additionally, people are less tolerant of dead pixels on a $400 laptop, and Pine64 would rather not deal with returns.