It is not a solved problem, unfortunately. That is why you see so much interest in the likes of PinePhone - even a rooted and de-googled Android doesn't really

It is not a solved problem, unfortunately. That is why you see so much interest in the likes of PinePhone - even a rooted and de-googled Android doesn't really give the same feeling of ownership and control as a standard GNU/Linux system. At the moment, the most practical thing to do is to pick whichever Android phone you find most physically appealing that also has LineageOS builds, scrupulously avoid any Google-made software (including and especially Play Services), and try to use it as little as possible. Keep the phone as a satellite device. Install KDE Connect so you can conveniently transfer photos and other data to a more suitable platform, and F-Droid so you can still tinker with apps. Expect to miss out on most of the usual closed-source apps - most are only distributed on Google Play only, and you can't have that without handing root to Google. Finding a phone with the physical properties you describe shouldn't be difficult, although expect to have poorer LineageOS support the more obscure it is. If you want a pocket device that truly gives you the same feeling as your desktop, and is immensely practical at the expense of not having mobile phone access, I can fully endorse the GPD Micro PC. Since I got mine, the phone only gets used for phone calls, navigation, and photography. I generally keep the phone on hotspot when I go out so the Micro PC has connectivity.