Ok - long story short - same result.
I now took a backup from the other router (r26446) as the base restore file for run u_fw (which was larger than all the other firmware packages, so the issue is not block size) - and it restored fine.
I then updated to r27240 via web update (unmodified web update file, clear settings checked) and it updated fine.
I then updated to r27805 via web update (unmodified web update file, clear settings checked) and it bricked the router (downloaded the firmware .bin several times, checked the md5 checksum, ...).
When I tried to update with any factory>ddwrt file from web update, the router simply wouldnt update (file gets transfered, router then rejects it and reboots - message in the browser says "successfully updated" - but thats simply the mask that gets triggered, the router doesnt update).
When I tried to flash the factory>ddwrt file with run u_fw (file doesnt start with the correct row of bits - in fact, this row is nowhere to be found) - which IS NOT RECOMMENDED (because it will most certainly brick your router) > brick.
But this time the error messages were different from the bricks before, complaining about magic numbers not matching and not having any valid firmware in any of the two "blocks" the router checks on bootup.
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So in fact, the brick I get when trying to update to r27805 seems to be caused by the update itself.
Sticker on the PCB says rev0.
This is the exact moment, when the router crashes on those newer firmwares:
And the most current Firmware I was able to successfully install on my european g450h remains
r27240
Sad to report it that way, but thats what happens in my case.