He's really bad for a 6th year QB.
I agree that on results he's a mid level 14-18 rank QB, because his athleticism, ability to run and flashes of great play lift him up.
As an actual quarterback, a position that is supposed to read defenses, go through progressions, have pocket presence, have timing, be able to make all the passes etc he's "really bad". Especially so for a 6th year QB.
If it was possible to get every starting QB in the league, give them the same west coast offense, the same O line and receivers, put them against the same defense and call the same 35 passing plays in an effort to rank their ability to read defenses, go through progressions, throw on time, remain chill in the pocket etc, all the things that are the core tenets of playing NFL QB, and he wasn't allowed to scramble, he'd be max 25th in the league.
Most of his production comes from one read or pre determined throws and his legs.