I agree with most of what you say except double concentration. That would make casters wildly unbalanced. The key to reigning in melee is to implement haste properly, so they don't get a full new action. If they only take two or three swings, they're much more balanced versus casters. I use a mod for this and it really do make a massive difference.
Casters in 5e don't outdamage fighters in 1v1 situations, nor should they. Casters are fine in AoE situations and for CC (once they fix the bugs).
