DALL-E 3 prompt engineering: Getting consistent brand imagery

I'm using DALL-E 3 to generate marketing images and struggling to maintain consistent brand style across generations. Each image looks completely different even with similar prompts.

Things I've tried:

Including detailed style descriptions ("flat illustration, minimal, pastel colors")

Referencing specific art styles ("in the style of corporate Memphis")

Using seed parameter (doesn't seem to work for consistency)

My prompt template:

Create a [subject] illustration in flat, minimal corporate style with soft pastel 
colors (light blue #E3F2FD, mint green #E8F5E9, warm gray #F5F5F5). Clean lines, 
no text, 16:9 aspect ratio, professional marketing style.

Results vary wildly. Anyone found reliable techniques for visual consistency?