Indeed, for edl it might be worth to try a usb 2.0 connection. This can be done by using an old cheap usb 2.0 hub if your computers do not offer any @thilo
`fastboot fetch` is just not available, sadly.
Regarding your backup attempts using adb, its necessary to root android using magisk to be permitted to access those partitions. Its an easy process documented here officially: https://docs.google.com/document/d/1sBS-Taw-3MXrG4lETA6dfuwda8K-9uVlzLhGk46ctzg/edit
Or in short,
- install magisk from the official source https://github.com/topjohnwu/Magisk
- grab the boot.img fitting to yo