Honestly, I think we're good on the left side.
Lindgren isn't a sexy player but he compliments just about every RHD we have well.
Miller is already doing what you are saying and is already playing 2nd pair minutes and PP and PK time, and despite his warts, is doing well all things considered.
Then you have your pick of Smith, Jones, Hajek, Reunanen, and Robertson for that last spot. Three of those five will not (or should not be) on the team next year.
I'm ok with Smith for another year under a million to be the 7th defenseman. The third spot should go to whoever earns it out of Hajek, Reunanen, or Robertson. All of them could conceivably earn it.
I'm much more worried still about the depth on the right side. Trouba being out exposed the inexperience of the group as a whole. I agree with
@bobbop that some veteran insurance be brought in, but I'd do it for the right side.
Say Trouba, Fox, and Lundkvist are your right side next year. There's Schneider, Skinner, and who else after that?