What should you consider before starting a SaaS project?

I’m planning to learn more about SaaS development. What do teams usually consider before starting a new project?
Should the main focus be on the product idea first, or is it better to plan things like scalability, security, database structure, and hosting from the beginning?
I’m also curious about how teams prepare for future growth. For example, what happens when the number of users increases, or new features need to be added later?
For those who have worked on SaaS projects, what would you recommend planning first?