There is no playbook for profitable bootstrapped SaaS in 2021. The winners either have a special secret that they wouldn't share for the world or they're lying about being profitable.. or in most cases the "successes" are simply such amateurs they don't understand basic accounting and believe that "profitable" means "slightly more monthly revenue than the cost of the servers".
If you embark on this be aware, the days of having your own little saas as passive side income are long behind us. This is serious serious stuff now.