Now we all want to know, if there is an actual thing in the code that suppresses that. I suggest a series of world wide tests because I fear for the sanity of tech support which would be the alternative.

30 GB in a backup takes a while, but it is also generally not a good idea. The bigger a file is the more likely that sometime during writing the file something goes wrong. I would recommend moving as many files as possible outside the backup ( enable sync instead for iCloud Drive, Photos, WhatsApp backup etc. that still requires the same iCloud space, but instead one big long process you have many tiny ones. That’s much safer and much less complicated.

Add the new card. Then remove the old one. If needed pay the outstanding amount using a gift card first.

I guess your iCloud space is not sufficient to make a backup and it hasn’t been for a year.