First off, congratulations! I think you’ve found yourself with an amazing opportunity.
First and foremost, always remember that you and your team are there to support that revenue stream. At the moment the junior developers have done that, but it sounds like they are at an inflection point and need help moving on.
Either one of two cases exists. The current state of software development is holding the business back from growing, or the business is near its limit, but the software is still a possible source of expense, reducing profit, either through excess maintenance or potential for failures.
In either case, your job is not to fix, it’s to lead and help.
First listen to each one of the developers in detail. Find out what they think are the problems. What difficulties they have on a day to day basis. Then teach them.
Perhaps they complain about losing work, or problems merging code. Teach them source control. Perhaps they really fear production changes causing outages. Teach them how to use a staging stack.
If the business is making revenue and sustainable, then you’ve got time and space. And always remember, that revenue is your goal along with your teams productivity. Your goal is not your own happiness with the stack.
If you stick with this company, the opportunity for personal and professional growth is incredible. You’ll learn skills you’ll use for the rest of your career.
So stick with it. And just remember, everything you know about how to run software development is the end goal. It’s where you want those developers to be at the end of the journey. But always listen to them first, and help them by teaching them how to help themselves.