Routes
What follows is a route for any% speedrun. This route allows you to finish the game by defeating as few bosses as possible, meaning that your primary means of levelling up is with Yeol. You must defeat 13 bosses along this route, which gives you at least 4.8 million souls provided that you solo all the bosses.
Extra souls can be obtained by doing an all bosses speedrun. An all bosses speedrun should provide you with enough souls to level up once, twice, or even three times in addition to the 5 levels you obtain from Yoel. The remaining 12 optional bosses together give you 6,734,950 souls assuming that you solo all these bosses. An all bosses speedrun would therefore net you at least 11,575,652 souls.
Next to each boss is the maximum amount of souls you would obtain after their defeat provided you were fighting the boss solo, equipped all the soul-boosting items, and you were fighting the boss in at least NG+7. The more phantoms you summon for a boss fight, the less souls you would obtain.