GrapheneOS is probably the best option out there.

As you said, it’s only for Pixels currently, because

1. They are more secure than most other phones. They have some kind of chip built in that makes them superior. I don’t know the specifics, but other commentators might add some information if needed. Something with encryption if I remember correctly. The GrapheneOS team is a bit …picky… when it comes to security, and most other phones don’t reach their requirements for a secure device.
2. Google is one major contributor to Android, and their phones are fine tuned to work perfectly with it. Other manufacturers’ phones feel less polished.
3. It’s easier to maintain one line of devices that are very similar, instead of keeping hundred phones up to date and secure. Pixels are similar to iPhones, they get updated almost simultaneously and are similar. If you now add a phone from a different line, e.g. a Fairphone or Nothing Phone, things get more complicated. If you look at Calyx (more onto that later), the FP4 caused quite some headaches for the dev team.

Pixels are cheap(ish) for what you get, and I believe Google makes them so cheap because 99% of users don’t care which ROM/OS is installed. Those are the advertisment-cows that will get milked. If you buy a Pixel and install a custom ROM on it, they will loose money.

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My experience with GrapheneOS has been great. My Pixel 5 hit EOL a while ago and still gets maintenance updates almost weekly.
Many security additions are overkill for me, but quite some make a lot of sense.

I used CalyxOS for a year too, but now that I don’t get full updates anymore, I don’t feel safe anymore with it.

I think GrapheneOS is technically superior to Calyx, especially due to the sandboxing they do. MicroG has full root privileges and can do with your phone what it wants, while also breaking some apps due to missing dependencies. If you choose to enable Play Services on GrapheneOS, they are user level and heavily restricted, and only you decide how much access you want to give them.

Regarding Calyx, since they don’t limit themselves as much in terms of security, they also offer a ROM for the Fairphone. Maybe check that out too.

DivestOS also seems to be a good option. AFAIK it’s based on LineageOS and supports a lot of devices, while being more secure than LOS.

Regarding Linux phones, I don’t have any experience with them. I tried Phosh (Mobile Gnome) on an exhibition a while ago, and it felt great and interesting, but from what I’ve heard, they are nowhere as good as Android.

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My personal ranking:

1. GrapheneOS on a Pixel. Get an used/ refurbished device if you don’t want to support Google. Best price-performance ratio, great OS, and very good hardware (battery life, camera, etc.)
2. CalyxOS on a Fairphobe. Modular device with good repairability. Nowhere near as good in terms of what you’ll get for your money. Better security than 95% of other phone ROMs, oh, and you can just swap your battery in seconds if you want that :D
3. DivestOS on a random supported phone, e.g. a China device. Nowhere near as sustainable (short lived update support, no spare parts, etc.)
4. Linux phone. Only a goof option for a tinkering device right now imo.