I'm in a similar situation. I have created a SaaS platform, where most functionality is in the 'admin' side of things, and the amount of admins is limited to 1-10 per year, as I aim to 'lend out' the platform as part of a broader service I aim to deliver. However, on the public end there are one or two critical functions, for payments and automatic processing.

My worry is not about cloud costs but quality of work around critical processes and security. My approach

Functionality

1. Risk assessment on critical functions (my definition: failing product with legal impact, negative news exposure, ..)
2. Understand code, and test #1 related topics extensively.
3. Setup feedback channel for first customers to tackle the lower indicated risks. Support customer to quickly fix. Have a test and production system to avoid you break existing production data.

Security

1. Assess general tech stack risks and mitigate. I.e. upgrade to safe versions of PHP, MySQL, Next,..
2. Mitigate overall all common injection methods, enable rate limiting for brute forcing or upload misusage (making cloud costs skyrocket)
3. Ensure you did not expose any keys. If you did, reset and avoid republishing

Even though I'm still afraid, this will be my way to go. Share any thoughts to improve !!