Best AI for image generation (Prototype/MVP)

Art is a huge bottleneck in the project and I would like to use an AI to create the placeholders so I can prototype quickly.
ChatGPT is unclear on the limits with their premium license, Midjourney is not private, I set up a local sdxl with comfyUI and it's meh.
What's the best premium AI for image generation with decent quality and high volume?