I have been using the Melin 808-e bit with a 1.5" cutting length and 4" oal. I do have to seat it almost all the way up in the router. You never want to push a router bit all the way up in the router. That can lead to an out of round bit rotating. I have cut a lot of deep pockets with that bit including 1.5" treated pine and it is still cutting like new. I have 3 of these total expecting the first one to wear out. I dont think you will find much longer cutting length.
With such a long bit be sure to set your retract height high enough to clear any clamps. I use painters tape and super glue so I dont have to worry about retract height because I have no clamps to avoid during rapid moved. Just dont set your retract height higher than your Z can go up so you dont skip steps.