Learn English – Word or short phrase to describe an individual who does not tend towards extremes of emotions

I'm looking for a good way to express the concept of someone whose natural predisposition tends towards less extremes of emotion than the average person; they get less sad when bad things happen, less happy when good occurs, less stressed during difficult times etc. I want to describe someone who still has real emotions, not someone who is unemotional or has such muted emotions as to feel unnatural, just with less fluctuation from their 'norm' than the average person. I'm looking for a term that generally has a neutral connotation, describing the trait as a trade-off rather than desirable or undesirable.

If I understand it correctly the original definition of stoic comes quite close to what I want, other than stoicism being about a philosophical approach rather than natural predisposition. However, I don't feel it conveys the concept as well as I would like due to modern use of stoic usually being applied only to enduring of hardships, which only partially covers the concept I'm looking for. While I would want to describe a person who would likely handle hardships better, due to not being as emotionally upset by them, I would like to equally express the trade-off of not being able to enjoy their prosperity/successes as much due to not tending towards extreme moments of happiness or other 'positive' emotions either. I feel stoic is too positive a connotation because it only brings to mind the useful ability to better endure hardships without expressing the trade-off of less enjoyment of desirable situations.

Ideally I would also not want to imply that this person tends towards an emotional neutral, or Vulcan, state—only that they don't have strong fluctuations from their 'default' state. Their default state may be happy, curious, morose, introspective, angry or some more complex combination of emotions, all that they do not tend to be as strongly shifted from their default by situations.

In the past the best phrase I've come up with is "happily stoic" to describe someone with a more positive default state; though "happily" sounds more like it's passing judgement on the utility of stoicism than defining their default state. In less exact, more jovial, terms I've used "an emotional Vulcan", which incorrectly implies a total lack of emotion and "anti-bipolar", which is close but only implies less fluctuation in happiness/sadness while I'm looking for a term which applies to all emotions.