Thanks, many people underestimate the power of older Photoshop versions. Sure, they don't have all the bells and whistles of the new ones, but I can still do a lot more on mine than most people can on gimp, including mass-automation. The downside is that if I have too much space on my hard drive the software assumes I have zero space on my hard drive and wont' boot up, so I had to install hundreds of gigabytes of games to get it to run. XD
Speaking of licensing. The weird part about AI art? You can't copyright it as-is. Companies cannot legally copyright works generated with their model. That's also why you don't see a lot of big companies (knowingly) using AI art because it could feasibly mean losing the rights to their IPs. That's why I see AI art more as the tool of the indie game dev and hobbyist rather than that of the big companies.
Also, here's a thought for you. There are some prompts that you can get on sites like prompt base to generate entire sheets of sprite art. That may help you in your quest to get 500,000,000 NPCs. I mean heck, even if you only pull about 3 sprites per sheet you're at least cutting those generations down to a third.
Anyways, take care and best of luck. Cheers!