Where should I (developer) as a single-member software company focus my time & efforts to build a current business into a real one with employee help?

My focus is business strategy and development, I am also a geek-at-heart! There are a few things to dig into to get rolling.
With your strength in tech - tap into virtual resources or even crowdsourcing to get the work done from a marketing and sales lead perspective leaving you to focus on what you do best.
I am happy to have a call with you to discuss steps! This is a critical time for your company and your passion is evident.

I am not sure I have any silver bullets, but would be more than happy to hear a bit more about your situation and offer advice. My company was in a similar situation a few years ago....so I can at least empathize.

I don't have specific advice about your specific question but do have some thoughts on deciding when to quit / scale or try something new (i.e. Opportunity Cost)

Most individuals make these decisions daily, but don't truly understand how to evaluate.

Here's my framework.

What's the potential upside? (Ex: $100K / year) x probability (20%) = 20K in potential.

If you think about that framework apied to other ideas like a new startup or getting a job, then you can better assess the opportunity cost.

For many people, making $40K/yr vs potential of $200K at 50% chance is actually more sensible then staying where they are at - but they decide to stay.

I always look at the opportunity cost of both new ideas + risk of distracting me from executing on existing tasks/projects - hence why I say "no" more now then ever (2 new babies + growing Startup)

Hope that helps.

You are WordPress developer so I assume you have well designed and well developed website. Have you seen any change in traffic patterns on your website ?

Take a step back and try to answer "Do my potential customers know about me?" "How will they find my product?". Answering these questions may open up options for what to do next.

Finally, read up articles by Neil Patel; he is a growth hacker worth reading.

Happy to talk further.

Do not assume you know your employees’ skill level and career aspirations. Some of your employees may already have development goals in mind, but do not know how to get started or if the company will support those plans. Other employees may not realize you see potential in them or need encouragement to reach for the next step in their career. She may remain reluctant until you help her understand that those skills are required to move into sales management, her ultimate goal. By talking to employees, you can work together to figure out what role your business can play in their plans as well as what opportunities you can offer them. Readiness comes in a variety of forms, encompassing desire, skills, and experience. Keep in mind that not every employee wants to – or should – move into management, no matter how good they are in their current position. Doing versus managing are quite different skills sets and pushing an employee into a job they are not ready for can have devastating consequences.
You can read more here: https://www.insperity.com/blog/5-steps-to-creating-employee-development-plans-that-truly-work/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath