Ask HN: How would you write requirements for a revamp of existing web app?

Funny that I anm asking this as I worked as a "Business Analyst" for many years in my corporate job life. Now I have a small bootstrapped startup that has an MVP working well with paying clients already. The problem is that I think it is not time to move on from the MVP and really build a scalable solution (yes, the market has been validated with paying customer already).

I want to write the specs for the functionality that my current application has and struggling for a way to do this. I hate to use plain old Microsoft word. Any other ideas to make it easy and seamless for me to communicate this to my developers ?

What's out there for something like this ? I know I can use Balsamiq for mockups etc. But how do I describe the UX ? How do I descibe the workflows ? Flowchart diagrams ? Sequence diagrams ?