Thanks Laurent.
No, in order to not get sync-locked, the Linux DT-app is shut completely down - only 1 single device is up with the Joplin Android app. I would keep it so, until all 3 devices - one after the other - would be synced-up and ready for normal work.
And no again, these are 3 devices each on its own 5G mobile connection.
Another funfact: I've also migrated another persons Evernote notes (ca. 3000+), also to a Linux Desktop, then to JoplinCloud and then to a Pixel6 Android. It took a long time for all the fotos and pdf, but eventually, everything was there and works as expected.
19:30 same day
In the meantime I found this conversation:
Might we be at a similar obstacle?
Link: 504 Gateway Timeout