Could you use a url parameter on the end of the real url (e.g. example.com/page?source=email_[date]), and then use the history object to strip the param once they land on the url (modern browsers)?

You're right, deliverability can be measured via pixel image itself. That is the same process that we follow at SendGlide.

Google caches those images though. So it isn't accurate for gmail.

Only for repeat opens. The original open still accesses a particular URL specific to that recipient so the open is recorded even if Gmail hides IP, browser, OS, and other data via their proxy. If the user doesn't view the images, that open is never recorded anyway. So, the caching doesn't actually affect the open rate.

They cache them before the email is even read. They also cache images across gmail accounts.