I grew up on SC Style and mustard was always my favorite. Personal preference is spicy and vinegary. adjust to your taste. My basic canvas for the mustard starts with taking the idea of taking it in size to your need rather than measures.
1/2 onion mince
Jalapeño or Habanero purée with some h2o & evoo
Sauté the onion and purée with another couple tbsp evoo and 2tbsp butter
1 part yellow mustard to 1/2 part cider vinegar for the base. 1/4 part beef stock or so adds some body. As sweet balance, I like to start with 1 part brown sugar.
Add this to sauté and bring to low boil / simmer for 10 minutes. Start with black pepper, salt, turmeric, paprika and cayenne in equal 1tbsp amounts initially. You can always add more but you can't take it back!
After it simmers for 20 minutes, carefully taste. Whatever you add, bring it back to simmer and let it sit for a bit so that it composes. When adding the sweet, I really stick with C&H Pure Cane and Light Brown mixed. They both add different dimensions to the mustard base. Hope this helps-