There isn't one, and pretending otherwise is nonsense because humans will always provide their credentials to something to act on their behalf.
In the limit you end up with Chinese phone farms.
Cloudflare, Google Captcha, HCaptcha etc. are all shitty technical solutions because, as we are all discovering, it comes at the cost of our privacy (i.e. our personal data may monetise these services) and / or our computing resource and time. If current copyright laws aren't sufficient to prevent this, we have to acknowledge the system is broken. The answer could be enhancing it with some kind of Digital Millennium Copyright Act (DMCA) -like laws, but in favour of the creators against BigTech or rogue actors.
- Web-scraping and copyright law - https://www.neudata.co/blog/web-scraping-and-copyright-law
- Why DMCA Claims Against Web Scrapers Face Long Odds - https://capstonedc.com/insights/why-dmca-claims-against-web-...
As for issues like bots overloading websites or using too many resources scaling laws will take care of it quickly, it’s not like you can’t serve thousands of RPS from a Raspberry Pi these days.
> we have to acknowledge the system is broken
The system is broken. It probably takes, what, 10 seconds or less to use a residential or foreign proxy, 6+ months to internationally track and prosecute a single offender? So like a million times more effort going the regulatory route.
Which sounds extremely difficult to differentiate
You can forget about it. It is not possible. Simple as that.
Then when it's "processing", do them in bulk and prioritize slower users. There's huge opportunity do bot checks after checkout without affecting user experience.
Also on product launches you could add unique field which requires user to input, for example that way bots can't prepare for launches.