(Disclaimer: I live off passive income from software projects and now run a VC-backed SaaS)
Repeating what others have said, starting a SaaS company broadly speaking does the opposite of what you hope. It gives you a job that you think about 24hrs a day, while maybe only actively working 6hrs a day, but realistically much more. That said I LOVE my job and super grateful I get to work on it everyday!
There are much better ways as a software engineer to take a lower risk approach to getting to financial freedom.
(These suggestions are not inclusive of coronavirus)
1. Save cash. Change jobs and increase your baseline income. If possible get to a FAANG and then also freelance on the side. It's very possible in the Bay Area after 5 years to save enough to retire without necessarily subscribing to the FIRE goals.
2. Work for enterprise companies or build them a micro-saas. There are plenty of high-value services that companies will pay $XXk monthly for. If your goal is to make money and maybe not love your job, this is a great place to be. Orchestrate Segment and a data warehouse, build out a robust Salesforce pipeline, take a spreadsheet and implement in Retool or Rails. Each of these examples can easily cost $300k for the right type of company.
A micro-saas example is Sidekiq ([https://www.indiehackers.com/interview/how-charging-money-fo...). I would have probably 4x the enterprise rate to get to $4M/yr.
3. Explain something difficult, really well. Related to selling services to enterprise, you can also consult with companies by owning a very specific niche that is very high value. Explain security compliance (like SOC 2), orchestrate edge servers and CDNs, up-level sales teams with technical knowledge or off-the-shelf tools. A single dev can shepard folks through a process that costs $XM of dollars or enable a teams that also costs $XM of dollars.
4. Rebuild existing tools, much better. This is my personal expertise, but I believe all existing tools could use major improvements. You can make tools better in UX, better documentation and tutorials, faster code, easier onboarding, etc. The major benefit is that you know there are existing customers paying money and you can then develop much better tools along the way. In this model I believe free or enterprise is way to go as a single dev. With free tools, you need strong traffic to monetize with ads or sponsorship. At the enterprise you sell featured postings or (3) services / consultation.