Unfortunately, it's hard to make Fairphone secure. No separate secure element (so much easier to do brute force PIN attacks) and always lags in monthly security bulletin patches and major OS releases (remember that the monthly patches typically only address high/critical vulnerabilities, for the rest you need OS updates, QPRs, etc.).

Until Graphene works out the deal with the OEM that they are talking to, Pixel is pretty much the only secure phone that allows installing alternative firmware.