Thanks for the insights and recognizing that appearance matters. Machines I have access to will handle a 19 needle.
I know that thread can be any weight a customer will pay for. (You and) I just listed a few benchmark ones that come up from the major makers with web presence (not inc Alibaba), benchmarks, maybe, because there is a US Gov spec at those levels, so there is lots of thread produced for the enormous appetite of the automotive and other upholstery industries.
I hope someone else who DIY's will look at their stocks of thread, and then at retail-made gear to offer some opinions of what they see.
This is just for DIY. And on fiber blends (and nylon): It was only from looking at catalogs that I saw that serious players offered them for outdoor use, no matter dogma about polyester for UV resistance.