I don't know if the suicidal ideation went away. They just became less explicit with it. All of the boys are vaguely suicidal. That's why they're so eager to sacrifice themselves. But I will agree that one of the most frustrating things about Castiel's wild ride is that even though I'm ultimately unhappy with where his character has ended up, there were a lot of Cas subplots and moments that I loved along the way.
I loved the introspection we get from him in The Man Who Would Be King. That scene where he's watching Dean rake leaves and can't bear to disturb his domestic contentment breaks my heart, as does the way Dean irrationally holds out faith in him until Cas' betrayal can't be denied. When Dean explained why Cas' plan was stupid and Cas asks him where he was when he needed to hear it he should have learned his lesson: that free will isn't just about groping blindly in the dark and sometimes you need to trust your friends to keep you on the right path (but of course, they couldn't let him actually develop post-S5).
I loved crazy Cas in S7. He was so sweet and innocent, and seeing him in that vulnerable state actually did show us a new side of his personality. It was fun to see him casually performing miracles, like making the Sorry board appear with a shake. They've never been that creative with his powers since. I also loved watching him struggle to adapt to human society when he lost his grace (although I will maintain that Endverse Cas was a more amusing and compelling take on human Cas). I wish they would have kept him in that state a bit longer, maybe given him a compelling reason to want to stay human before he's forced back into angelhood. So many great moments and lost opportunities.
I think he really started losing me in S8, when he bolted with the Angel Tablet for no discernible reason, and then kept repeating that same behavior over and over. The thing with Jack in S12 was the final nail in the coffin. And the debuffs are just lazy.