Is dukkha conventional or ultimate? Is it different from the other two marks of existence? Is the existence of dukkha a conventional or ultimate truth?

I am confused, because on the one hand, it is usually mentioned together with two other marks of existence, which clearly look like the ultimate truth: "self" is conventional, and ultimately there is no self, and permanence of phenomena is conventional too, and ultimately everything is impermanent.

On the other hand, dukkha looks like something conventional: suffering, or lack of satisfaction, depends only on the existence of the sufferer, and ultimately, there is no sufferer, there are only empty aggregates.

Moreover, if I were the last sentient being to become enlightened, there would still be no self, and phenomena would still be impermanent. But there would be no suffering. I don't know how to wrap my mind around this asymmetry.
According to the Wikipedia article Four Dharma Seals, some schools of Buddhism teach "Three Dharma Seals" as including Nirvana but excluding Dukkha (for example Google finds The Three Dharma Seals by Thich Nhat Hanh) because "suffering is not an inherent aspect of existence".