See Four Depths of Faith and Five Stages of Practice.
Nichiren believed that he was, on one hand, at the stage of "Hearing the Name", the second of six stages of development.
Alternatively, he also speculated that he was among the Bodhisattvas of the Earth, if not Jogyo (Visistacaritra), which would mean he was an advanced bodhisattva - far beyond development of even Manjusri, Samatabhadra, Maitreya, etc.
These two positions are not really in conflict from the perspective of the Sudden and Perfect. Just as Shakyamuni displayed birth, awakening, teaching, and then death, so too are the people who practice Dharma merely demonstrating expressions of awakening, namely the path to awakening/cause of awakening, for the benefit of beings. This makes the cause of awakening really the effect of awakening... its one of these regressing rabbit holes of logic we tend to find in Dharma demonstrating the immeasurability of reality.
Grok that. We are, as it is, in the realm of immeasurable, the boundless, the unarisen, the uncreated, the inconceivable. The limitations we perceive are arbitrary and fictional, while still carrying effect. Grokking is the point. Meditation, chanting, study, contemplation, reflection, ritual purification, etc. these are means of preparing the mind to grok. In the meantime, we accept it on faith and proceed accordingly taking real action in this life confident that what the Buddha teaches about reality is true.
But back to the stages - Nichiren also implicitly accepted the 52 stages of enlightenment at times utilized by Zhiyi. Zhiyi asserted that he wasn't even on the first stage, and was still at the second or thirds stage of preliminary purification. I don't recall where Nichiren placed himself on that measure of advancement.