What are you familiar with?
Does it have easy full disk encryption?
The National Cyber Security Centre publishes assessments of different operating systems; you might do well to take a look through those, and see whether you are comfortable with the degree of security for the risks that you have identified.
Physical switches for various functions.
May be able to remove network cards / Wi-Fi boards if you really want to do so.
Read chapters 1 and 2 of the excellent “Information Security for Journalists”.
How quickly will you get security updates? For how long?
Are you stuck on an outdated version of the operating system? You might be able to install an unofficial third party firmware, but unlikely to obtain any formal customer support if you do so.
Is the phone unlocked — can you put a different network’s SIM into it.
This is not particularly security-related, but it does give you a degree of additional control if you decide to move providers.