With my utmost respect to the other posters and their suggestions, I only say this from my perspective.
Don't let security be the determining factor for you. Instead consider which hardware and what software options you will have, and how "robust" the privacy is. I don't know how much better that one plus phone is compared to a pixel. Nor do I know any of the features and privacy elements of DivestOS. But I can tell you after I considered a number of OSes, what made me go with graphene wasn't mainly the security ( that was a bonus). It was the the price to start, software compatibility and the amount of flexibility it offered. Because of its Sandboxed Play, it means I will not run into future "FORCED TO HAVE" apps not working.
- Total google independence
- Sandboxed google play (strong app compatibility) (even MicroG an option - not recommended)
- Profiles (separate contacts, apps, multiple accounts running at same time, saparate VPNs)
- Very "robust" privacy
- Total network denial to individual apps (huge deal, makes a big difference - not available on any other OS)
- Sensor denial to apps
- Original phone camera app working (no compromise in camera quality)
- mac randomization
- top of the line security (better than stock)
- 5 or more years of life
- More setup required at start
The pixel has these issues though: (6a)
- No phone jack
- touch registration not 100% all the time.
- no external memory
- storage only 128
Anyway, Consider whats important to "YOU" and think of the future, your wallet, and realism. Actually really think of your app needs and their compatibility and your privacy goals. Don't fall for "better hardware" hype-trains either.
I'm blown away how close this phone is to a non-privacy phone and little compromise there is. I'm glad I didn't go with some of the other systems.