The followings are Thay's advice and my commentary.
"Being able to enjoy happiness doesn’t require that we have zero suffering. In fact, the art of happiness is also the art of suffering well. When we learn to acknowledge, embrace, and understand our suffering, we suffer much less. Not only that, but we’re also able to go further and transform our suffering into understanding, compassion, and joy for ourselves and for others."
The above is based on the conventional truth in the historical dimension (phenomenal world) because suffering and happiness are regarded as two separate entities. That is the duality. So, thinking is still there. Only while we are mindful, we can stop thinking and recognize, embrace, understand and transform our suffering. We need to remember that The Four Noble Truths is also based on the conventional truth.
"As soon as we open our mouth to say “suffering,” we know that the opposite of suffering is already there as well. Where there is suffering, there is happiness."
Meanwhile, the ultimate truth in the historical dimension is that suffering and happiness are two sides of the same coin. They are interdependent co-arising. So, suffering is no other than happiness, and happiness is no other than suffering. That is the non-duality based on the duality.
Furthermore, the ultimate truth in the ultimate dimension (noumenal world) is that suffering or happiness is impossible without separation caused by notions. That's because of the extinction of all notions in the ultimate dimension. There are no notions such as suffering and happiness. That is nirvana. That is the wholeness. That is the genuine non-duality. So, in this state there is no thinking at all. We need to remember that The Heart Sutra is based on the ultimate truth.
"Why the Buddha Kept Meditating?
When I was a young monk, I wondered why the Buddha kept practicing mindfulness and meditation even after he had already become a Buddha. Now I find the answer is plain enough to see. Happiness is impermanent, like everything else. In order for happiness to be extended and renewed, you have to learn how to feed your happiness. Nothing can survive without food, including happiness; your happiness can die if you don’t know how to nourish it. If you cut a flower but you don’t put it in some water, the flower will wilt in a few hours.
Even if happiness is already manifesting, we have to continue to nourish it. This is sometimes called conditioning, and it’s very important. We can condition our bodies and minds to happiness with the five practices of letting go, inviting positive seeds, mindfulness, concentration, and insight."
Happiness and suffering are polarities (both extremes), not peace (Middle Way). They are just the notions. That's why I feel that the Buddha must have preferred peace rather than happiness. And the method to attain peace is meditation. Through meditation (stopping, resting, calming, healing, nourishing), the Buddha could attain immense life energy (mindfulness, concentration, insight, compassion) and peace. That's why the Buddha must have kept practicing mindfulness and meditation even after he had already become a Buddha. The Buddha must have wanted to rest in the ultimate dimension, nirvana, or peace.
Thích Nhất Hạnh