Also, I know the intent of what you're saying when you say "passive income" but after ~8 years of being obsessed with these ideas I offer a slightly more experience perspective when I say You probably aren't going to earn any "passive income" from your Saas. Let's say you get 30 customers @ 20/month. That's $600/month but you shouldn't appropriate that as passive income if you're still building features, performing service maintenance, etc. Maybe if you freeze the product and don't perform any marketing work, server work, etc. then it's "passive income".
In other words, you should focus on building a business, not a passive income. If you invest money in bonds or dividend-distributing stocks, then that's a passive income. Because you literally do not have to do any work or stress over it if you take an extremely low-risk, highly-diversified, income-producing investment strategy. How do you get the $$ to invest? By building a business and investing lots of energy (the opposite of passive).
Can you build a business while working a job? Yes, it's done all the time. Another tip is to wake up every morning very early and put your best cognitive hours into your project. Then you can go to work, and by the time you're ready to get home you can fully invest in being present.