The having another woman to vent about you certainly contributed to the arguments. Something similar happened to me in my marriage.

As I'm typing this, I forget if you mentioned if he's seeing a counselor? He tossed out there about wanting to die, which is concerning on two levels in my mind. One the obvious with depression and ideation. Two the fact that statements like that said in moments like this can be manipulative in the sense of controlling you from possibly considering leaving, guilt inducing. It's important that he knows you will contact someone/crisis team/emergency ambulance if he gets like that again. (it truly restores balance and gets them someone to talk to. As much as one might want to not be in that scenario it's important) I cannot stress enough how important being that firm on that is. I've done it myself. It also releases a weight for lack of a better expression with everyone involved.

Date nights seem like a good step. I hear you about 'now that my oldest is old enough' statement.

I think OE is onto something with the mothering aspect, especially more so with your reply.