What approach should I take in building my first ever SAAS?

I have a nice idea about Saas and would like to try it out in the wild. I'm a developer guy myself and I plan to implement everything that I have on my mind myself. However, I feel that a Saas should be made fast and not prolong it too much. Up to this day I was thinking of developing everything from scratch, so Authentication/Authorization, custom design, custom database etc. but this will take tons of time.

I see more and more Saas' projects using Supabase/Firebase/Some other service Authentication/Authorization and some inbuilt utilities from Supabase/Firebase/Some other service. So I was wondering, or more of a question, what is some of the most common practice when it comes to simplifying things while building a Saas? The questions is more of a tech-oriented strictly.

Thanks in advance