How to track mail undetectably?

Is there a way to track emails, so no one can detect that emails are being tracked?

No.

If email is tracked it must communicate that information somewhere and therefore must leave a footprint of some kind.

You can use such tricks as a unique image (i.e. the image is only in the email to one recipient ever) and watching the web server to see if the image is accessed, but this will only work if the user loads the images in email, which doesn't always happen, which you've experienced with thunderbird. How much content is loaded is decided by a users settings and so isn't something you can control.

Some companies will provide a link in an email 'can't read this email? click here to view online' which is basically the same thing, but instead of a image online they present the entire emails content again, under the guise of 'helping you' when in fact it's all about tracking you.

Short answer: No.

Longer answer:
The feature to load remote content only after the user allowed it, is mostly implemented to protect your privacy. Of course there are effects like less data usage and maybe some people do not want to have so many images in an e-mail, but the main reason for this feature is data protection.
When there would be a way around this (a feature for you as sender and person who wants to track the e-mail), this would be a bug in the protection feature and the mail program developer would hopefully fix it as soon as possible.

So what you're asking for is a hack or exploit, that allows you to circumvent security measures in the client software. There may be answers for specific softwares, but all of them would be security bugs in the software and may get closed sooner or later.

Another remark:

Is there a way to track emails undetectably, so no one can detect that emails are being tracked?

Don't you think, that this behaviour is kind of sneaky? It may even violate GDPR. If you want delivery reports, just ask the user for it.

There is even a feature in the e-mail standard for it, that should be GDPR compliant, because the standard says, that e-mail software should ask the user before sending the report. If you want an unified UI across different mail programs, add a link "Please click here, to confirm that you received the message".