I have mainly focused on full custom interiors and have recently realized it isn't worth it (especially in my area). The majority of my customers are on tight budgets and cannot afford full custom interiors. I have really hurt myself by lowering prices, and in the end, I make no money. You tell someone its going to be $8-12k for a full custom interior with premium leather, they about faint. People usually have no idea the amount of work/time it takes and skill sets needed. You need to be a competent carpenter, body work, metal fab and then a trimmer. Now that the economy is getting squeezed, people's projects are getting put on hold. I have had quite a few people call and have to cancel due to finances. I totally understand that as well. So, small jobs are king now. Even then, material costs skyrocketing are starting to make people balk at having work done.

https://www.youtube.com/watch?v=4TH9rSqPWAs

https://www.youtube.com/watch?v=WErjiKMOq2A

Those videos should give you a pretty good starting point.

https://www.amazon.com/Custom-Auto-Interiors-Don-Taylor/dp/1931128189

That book is a killer reference to all things custom.

What size thread are you using? 23 needle is pretty big, especially if you are using a smaller thread. Ive had this happen before. The needle makes too large of a hole and the thread is too small and cannot fill the needle hole. That creates space and shows the stitches.

Do yourself a favor and buy all of Sid Chavers DVD's. Everything you need to know to get started in the custom world is there. He walks you through how to build seats from scratch.
https://www.sidchaverscompany.com/product/sid-chavers-upholstery-dvd

Windex has always worked for me and if it has been on there for a couple days use lighter fluid. The little bottles that you would refill a Zippo with. It has never hurt leather or vinly.