Boicy
It depends on at least 2 factors:
- What application software you want to run
- How flexible you are about your security model
Based on your request, you are probably still thinking about mobile apps, for that you might actually want to stick to a proper mobile OS (Android or iOS) rather than a mobile linux phone.
If your security model is very flexible you can consider Uubuntu touch et similia, which will bring you amost all pc vulnerabilities in your device.
If your security model is flexible (but not much), you can consider some Android distro.
If your security model is not that flexible, buy a used iphone.
Avoid root or jailbreak.
If you already have a Pixel <6, consider PixelOS