In our case, our "start-up" is a desktop product sold as a subscription that is developed and marketed within an existing professional services company we operate, which is serving a niche domain that the desktop product targets.
Developing the product was always a bit of a hobby but patiently over 5 years since it transitioned from freeware to commercial, annual subscription billings have grown to over $500K. We estimate that our current customer base is a 3-5% of the TAM. The product business itself is already profitable because it requires barely one FT equivalent person for development and support (i.e. 2-3 people part-time working on it, mostly dev, a bit of support).
Since it has a hobby, we fully relied on a low touch sales model so we never have done ads, conferences or engaged in sales efforts. Instead we grew via network effects among our customers.
What I'm trying to say is that in all these years we've figured out some things along the way (most notably product-market fit and business model) and clearly left other things on the table (sales, marketing, recruiting) to fully develop the opportunity further.
Back in 2014 I found immensely useful the "How to start an start up?" video lectures hosted by Sam Altman.
Given our slow motion, age (I'm over 50) and probably the fact that our company would be an outlier in the group calls, what would make most sense for me to apply for, Startup School or Audit Startup School? Of my team most likely only I will have enough time/motivation to take the online course.