So it feels like there's some other agenda here, but I can't figure out what it would be.
Nope, it's exactly what it looks like. It's marginally-targeted spam. The sender is a scummy SEO marketer, and is hoping that you will allow them to write a (garbage) "blog post" for your site, filled with backlinks to their content.
If they really thought I'd like ...
Yes, it is spam.
I really doubt you have legitimate customers that subscribe to random forms using headless chrome.
No, it is not enough to block the user agent at nginx level. It would be trivial for an attacker to change the user agent, and even more so with your implementation to see you have blocked them.
At least, had you filtered it at the PhP file, ...
From my perspective this sounds like someone is trying to deliver some spam to you. I don't know why someone is doing this and what's his purpose but it looks like.
Regarding your question:
No it's not enough to deal with it that way. It's very easy for an attacker to alter the user agent and move on like before. I would strongly recommend to use ...
You are probably safe if these are true:
You didn't click the link*
Your browser is updated with the latest patches**
You aren't using any chrome or browser extensions***
Your computer is updated with the latest OS patches****
Generally, most exploits utilize well known vulnerabilities in common software like your browser, operating system, popular ...
Increase site ranking
When search engines parse pages (Googlebot for example), they look for backlinks in order to try to determine the popularity of a website.
The more backlinks go toward a website, the better search engines will rank it and the higher it will get in search result.
Decrease competitor site ranking
It's also possible that the attacker ...
it is not dangerous unless you open a link, or open any kind of attachments. of course vulnerability in Google calendar is another issue. (like zero-day exploits that are out of your hands).
in general enable two step security on your account, install update asap, and you are ok.