The correct behavior is to be able to request a marker when signed into any email account, and on my side set the tag that it gets tagged with in that inbox as a result. The link between marker and inbox should remain secret.
500 is a reasonable limit I think, in case spamming would be some reason for them not to do this. I don't have an opinion about whether people should be able to send from marker@gmail.com or if it should just end up in a real inbox but without the ability to send from that address.
If Gmail gets compromised, you'd need to be looking for a bunch of Fastmail accounts in To: addresses to link my primary email with those emails.
If you wanted to track me down from an email address, you'd need a warrant in Australia (for FastMail), and the US (to find the account using those FastMail credentials), so I'd need to have actually done something wrong (which I haven't), and you'd need to convince judges in two jurisdictions of that. As I said, the threat model is against casual snoopers, rather than a determined state actor with proof of wrong-doing, as I don't think I'm even slightly interesting and I don't think I've done anything that would make me interesting.
As it turns out, you could probably just read enough of my FastMail email as it came in (before it gets deleted by Gmail) to figure out who I am, so this is imperfect.
[0] https://github.com/ietf-wg-acme/acme/blob/master/draft-ietf-...