Despite having little to no experience in your target industry and no product yet, you have already lined up paying customers. From what you indicated, many. Most entrepreneurs would envy the position you're in.
Your next steps are to delight the customers that you already have with an MVP, learn from them, and delight them with a product that keeps them coming back. Because you already have quite a few customers lined up, developing a scalable back end fast is important for success.
Sit down and make an honest assessment of what you can really do with the resources you have right now. What type of product could you actually build? How much support can you actually provide?
Make sure what your customers can expect from you now vs later is clear. For the customers you can't serve right now, let them know when you think you could help them, and proactively communicate about your progress and roadmap to keep them engaged.
You need to build a product fast. Luckily, there are a lot of low code and no-code solutions that can help you automate simple tasks (like Zapier) and create an minimum viable product in hours or days rather than months. Check out toolmeup.io for a comparison to find the right tool for you.
Are these the best/cheapest options at scale? No. Will they help you grow and learn quickly? Absolutely.
Learn as much as you can from your customers, and turn that into a superior customer journey that solves their pain points without creating new ones.
Constantly experiment - whether with A/B testing on your website or product, or simply asking your customers for feedback on their experiment - and turn your findings into a seamless front end.