Does it still seem to be indexing or is it still kinda warm/hot? If it’s cool, it’s probably finished, I’d say to reboot it then. Indexing may be taking a long time, too, due to the lack of free space.
How much free space did you have before trying to update, just curious?
It normally had 110-120 GB free.
With 26.3 it already completed. Absolutely cool now
iOS 26.3

Compared to 26.4 after less than 24h:

It had 0.86 GB free when I decided to restore again 😫
It was warm on power but pretty cool if not. Settings told iCloud backup complete.