Hi @Ralf, sounds like Etcher doesn’t address your need at the moment. If you feel strongly about this, I encourage you to open a feature request in Etcher’s repo: https://github.com/balena-io/etcher/issues
This way you could talk to the repo maintainers directly and also create visibility for Etcher users & developers to chime in. If there is enough interest, perhaps Etcher team would consider your feature request in future versions.
Additionally, I’d also encourage you to talk to PureOS developers to make PureOS setup more convenient for OSX users who are not able to disable EFI. In my humble opinion, your problem is quite specific and it goes beyond what Etcher promised to help with currently.