I hesitate to weigh in on this kind of conversation because my comments may come off as self serving. Hopefully this will just be additional food for thought to anyone looking for interior options.
I think the biggest benefit you get when you go with an established RV interior supplier is the time spent designing the products before you ever place your order. The RV-14 side panels for example took over 6 months to design (including many late nights). In that time things were refined and adjusted many times.
We built a full scale cockpit mockup with parts from Vans for this process. Each one of our parts are designed in solid modeling software and then manufactured to precise specifications. There are hundreds of parts that are laser cut and formed using a CNC press brake. Small details like attachment brackets and installation templates are all included. Over the last 19 years, we've learned a lot about what works, and we're still learning.
It depends a lot on what you are looking for. For some a traditional upholstery shop will be a good fit, for some it won't. I wish I could snap my fingers and reduce the lead times, but the changes needed take time. We're working on it though.
PS: Any upholstery shop can order materials that meet FAR 25.853 along with lab test results. Also most non-aviation flammability specs are far less stringent. If a shop doesn't have material options that meet FAA specs, you could direct them to companies like these: