I'm trying to promote more coat length, and just wondered if anyone has tips that could help. Thank you.

In my experience its genetic, if parents etc have good coats it will come with time. I've often despaired of young dogs (borzoi) with thin, short coats, and over the years have tried every supplement, food option, shampoo etc that people recommended, time solved it in every case bar one, he never developed a heavy coat, but then neither did his Dad.

Coat texture appears to be different, I've noticed my current silkens coats change texture depending what they're fed. Wolf of wilderness has made for amazing soft coats in them all and I know course coated dogs can be affected by over washing etc

What breed?

I've never found anything that would encourage growth, but you can definitely discourage / ruin a coat by over bathing or over grooming.