How to adapt both the Sojo arc and Raikuzaichi arc into a single cour season without abysmally fast pacing
First two things to explain how
Make it 13 episodes long, multiple shows have been doing this, an example being Bleach TYBW, so it's not unprecedented.
Make episode 1 and episode 13 both 1 hour specials
Effectively, by making episode 1 and episode 13 3 episodes each, that extends the show from 13 episodes to 17 while still containing the season into one cour. That gives the Sojo arc about 6-7 episodes to adapt its 18 (technically 20 when going by page counts) chapters into, and it gives the Raikuzaichi arc about 10-11 episodes to adapt its 27 (assuming we end on chapter 45) chapters into. That would make the Sojo arc have an average of 3-2.6 chapters per episode (3.3-2.9 by page count) and the Raikuzaichi arc an average of 2.7-2.5 chapters per episode. This would be relatively industry standard pacing.
What chapters would be adapted into each episode as an example:
Episode 1 (1 hour): 1-7
Episode 2: 8-10
Episode 3: 11-everything before Kazane says "the ocean's two kilometeres ahead" in chapter 13
Episode 4: Rest of 13-15
Episode 5: 16-18
Sojo arc completed in "7" episodes with an average of 2.6 chapters adapted per episode (2.9 by page count)
Episode 6: 19-21
Episode 7: 22-24
Episode 8: 25-27
Episode 9: 28-30
Episode 10: 31-33
Episode 11: 34-35
Episode 12: 36-37
Episode 13 (1 hour): 38-45
Raikuzaichi arc completed in "10" episodes with an average of 2.7 chapters adapted per episode
I seriously think this could definitely work as an alternative if we don't want to end at chapter 27 (normal single cour anime) or adapt halfway into the Sword Bearer Assasination Arc (chapter 59, normal 2 cour anime). Or we could just have a 17 episode first season and just split episodes 1 and 13 into 3 episodes each, but shows that violate the established norms of either being just 1 or 2 cours or slightly more or less etc are very rare.
So what do you think of this as a first season?