One part of Agile that may help here is continual incremental delivery.

If you can deliver new functionality to your client each week, a little each time, then they should be happy to see progress, and each week you could re-evaluate how many weeks there are to go?

Just a thought.

When I was getting started, I would often obsess over the "right" formats for different types of communication with clients.

Ultimately, I decided what clients really want is to know you are doing everything reasonably possible to take care of them, being courteous, keeping them informed, yet not drowning them in detail.

So, the simplest format that covers these points is a good start. You can then ask for feedback.

Acunote is what I use to plan out my sprints. It does all the stuff of creating charts (which clients always like) and predicting how much work you'll be able to get done with each new sprint.