Mael There is still no solution or application to have a different password between boot and lock?
Not yet, but there is some recent activity on issue #28.
Mael How long should the password be to have very good security (digit with Titan) ?
Fundamentally, longer is better, and different people are concerned about different threats. Some people are worried about a randomly stolen phone, but others are worried about nation-state actors with supercomputers -- those threats are very different.