Earlier quoted context omitted.
Hopefully you are successful, but how can you scale? If it takes 5 seconds on a desktop, then a server can solve 500.000 captchas per month. At $5 per month, a spammer can still send 1.000 messages for a cent.
It's not enabled yet in production - but the main mechanism is by increasing the difficulty as more requests are made from an IP in a certain timeframe (it's basically rate limiting at that point). Think: every 3rd request in a minute doubles the difficulty with some cooldown period. With that the cost (and complexity) of an attack can hopefully be in the same ballpark (or higher) than ReCaptcha - without your end us…
[1] Some examples: https://rhinosecuritylabs.com/aws/bypassing-ip-based-blockin... https://oxylabs.io/products/real-time-crawler https://github.com/alex-miller-0/Tor_Crawler https://www.scrapinghub.com/crawlera/