Basically, you are 20 years too late

I know of no CRM or mail software that uses any other method...

For example, we use Send In Blue for some transactional emails and use their email tracking. They add something like this to their emails:

<img width="1" height="1" src="https://fgfcgfj.r.af.d.sendibt2.com/tr/
+op/ubVEi9KMORTJR7DtkZjC4DjoU--GFjX4L8Ive0tmrRHobDcJWwFSnv4PdLwO4VRiZ1
+6P1arvc-oAwLKIFN_U8zi7OcP99UZXHA7bwdO0_yAM3yvQPuk3ocgXu2pOprxjHTynTTj
+cPblGap6rVGNweeM1Wro2dAoIK-xRqlhhQGGGN8" alt="" />
[download]

Having a user click on a link is a more precise trigger of action as users and mail software can affect opening data but we still need to know who has definitely opened an email to be able to personalise relevant content. The data we collect in email processing is not personal data as defined by GDPR and our data processing systems are registered with the Information Commissioners Office as required here in the UK.