Fan consensus: S4 (Chen Du arc) is the peak so far.
No, but it helps. The show explains most concepts (e.g., Qi, Yin-Yang, Taoist rituals). Hitori no Shita- The Outcast
Crunchyroll (seasons 1–4, partial 5), Funimation (earlier seasons), Bilibili (global). Fan consensus: S4 (Chen Du arc) is the peak so far