This may be an unpopular opinion, but I would actually be in favor of keeping Diaco on as the defensive coordinator (assuming there is a new head coach next season.)
Say what you want, but I believe there's been major disruption in changing the system, hearing from a new voice as the D coordinator, a new tackling method, likely not the right skill set for a 3-4 system, etc.
What I've mostly witnessed is a team that seems to settle down in the second halves of games. I think over time, when a team is more comfortable in a scheme or system, they won't need that settling down.