There's been enough zero day remote exploits that there's bound to be more.
Pretty sure there's more than 1 about receiving an SMS and the payload rooting the phone and you not even knowing it happened. At least 1 but I think 2 or more.
Something about a malicious image also rooting a phone.
It goes on and on and phones don't always get security updates.
You can do your best, but then longer you use a given phone the higher the risk. That's why people switch out phones frequently when doing shady or important shit