When I built my little side project (B2C iPhone app, not a SaaS [1]), I documented the development process on LinkedIn with short "developer diary" videos. That has led to a good amount of engagement at launch, which briefly lifted the app into the top-paid section on the App Store for a short time. That was enough to reach the first 100 users within a few days.
Since it's a side project, I haven't worked on the app in a while, but recently picked up development again. So if you have any ideas or suggestions, they are very welcome.
[1] https://apps.apple.com/app/dorepeat-checklists-todos/id15615...