I agree.
I think Zev could cope with just about anything, he could deal with being a Warden and not be too miserable about it. But Cullen as a Warden makes me sad
He would be a great Warden and would never shy away from his duty, but like you said it really would suck the spark out of him.
I think that a normal life would make Cullen the most happy- just having a place to call home, a person he loves and a job where he feels he's doing something that's right and important.
Personally, I don't see Cullen as Warden material.
As strong in his convinctions as he is and committed to his duty, I don't see him as being someone who could truly commit to that way of life, not in the same way as those like Duncan (at least when he got older), Oghren, Sigrun, Carver and the Warden seem to, who understand it's a lifelong commitment and that they'll most likely end their days fighting Darkspawn in the Deep Roads.
He's more like Anders, Alistair and Velanna, who joined the Wardens either through necessity, ulterior motives or simply to escape the Chantry. While they were good Wardens in their own right, they never really in their heart wanted to be doing that job forever, or would have chosen it if they had another option.
Unlike what Hackett said In ME3 about Shepard's belief in their cause making them a great soldier, I don't think that Cullen would be a believer when it comes to the Grey Wardens. He might believe in the Templar Order, but at the moment it's more in the idealised Templars that exist in his head, not the Order as it currently stands; something I think he woke up to at the end of Dragon Age 2.
Cullen's got a great strength of character, no question, but I don't think he's got the precise amount of character that being a Warden requires.