These bots are common to your visit as they crawl your site to get your content indexed. Maybe we could try another technique. Let me think and see how I can help you.
Give me specifications of your server, if it is shared, dedicated, vps... so I can know something more about it.
Maybe we could use CloudFlare on your domain dns.
I am quite specialized in this, although this cloudflare is not 100% protection, although it can stop many attacks, in this case, this one.
Your attack follows a pattern, you would simply have to track that tracking and see if it changes the IP or it is a bot, from here restrict and add rules to block this.
Although doing this, I like to make an in-depth analysis of what this attack is due to and what is the reason to avoid more related problems in the future.