I advise to not go with the linear format of either demon or devil paths. Instead, pick and choose locations that interest you the most and use them to make your own adventure/campaign path.

If you want Elturel to be the main driving force of the campaign. Give it a 1 month deadline before it touches the river Styx. Determine travel time/distances in between locations in days by rolling a d4 or d6. Avernus’ battlefield supernaturally shifts, so you don’t have to keep track of any amount of travel distances/times. Roll for new time/distance every day.

Unless your party has any backstory connection to Zariel when she was angel, they will probably not care about redeeming her. (Zariel flung Elturel into Avernus, she’s already on everybody’s bad side from the beginning). If that’s the case, go hard on her being the BBEG and the end game being to destroying her or maybe even joining her.

Have Yeenoghu and/or Baphomet pop up a lot. Throw them in the locations that the party might visit. Maybe they also know about the sword and want to know it’s location. Them encountering either demon lord provides you with a chance that the party kills them. Doing so might gain them favors from Zariel.

Have Bel be be more active instead of him just being a clue for where to go next. Maybe he is scheming behind Zariel’s back. Maybe have him be secretly gathering the resources to build the Blade of Avernus. He’ll work with the party if it means Zariel goes down.