Just to follow-up on the original post (@Moem, @danfro): the Signal bridge bot at tchncs.de currently works again . They managed to block the spam.
A single addition. The Signal bridge bot replies with a QR-code and a sgnl://-URL. My primary Signal device had some trouble scanning the QR-code - it got picked up as I was just about to give up.
I suppose the URL can also be used for linking the devices, so if anyone has done that successfully, it may be worth posting here.