Nice to read your device is now fully working ! 
The link is missing on the page, but available : https://images.ecloud.global/stable/GS290/recovery-e-0.18-q-20210827132306-stable-GS290.img.sha256sum
I created an issue : sha256 sum missing for /e/ Recovery (#3995) · Issues · e / Backlog · GitLab
About fastboot : I guess you’re referring to the binary from SDK Tools ?
It addresses directly to the firmware, so it may be a firmware problem.
As I can understand, you had no problem using latest fastboot with stock ROM (and firmware).
So, the question may be : why is /e/ using an old firmware ? Or : why didn’t Gigaset publish the latest firmware ?
I don’t know enough of the GS290 /e/ building process to reply to this myself 
Fastboot is maintained by Google, so it’s unlucky they feel concerned about AOSP custom ROM 