There are a lot of devices with the ability to install another OS and lock the device with verified boot, but none with the required updates and security features other than Pixels. Fairphones are near the bottom for security among the available options.

It's not one of the main issues with their devices but Fairphone has had a lot of issues with verified boot including using publicly available sample private keys for signing firmware and OS images across multiple device generations. It's not a strength of their devices.