My solution was similar. Anything coming from the ASN of a major cloud provider gets a CAPTCHA with a little nuance to allow Google and Bing to index. That seems to do a pretty good job. Also, anything coming out of China or Singapore also gets a CAPTCHA as my site is not popular in those regions and many Chinese bots seem to show up as a Chinese mobile provider. So far, the bots have never attempted to solve the CAPTCHA.