I have an older MTB no suspension, cromoly double butted frame. This winter I put on a set of trekking bars, a quill to non-threaded adapter and an extension to raise it all up. A new set of touring tires and this is the most comfortable bike I've ever ridden. Keep in mind I've never ridden a carbon bike.
bottom line, I'm wondering if with a few modification, your hard tail MTB might be wonderful? I also own an aluminum hybrid that I rode last year that rides like a buckboard in comparison.