I don’t think that associating the entire 60gb patch with Siptah content is entirely accurate.
When a game is updated, you often have to re-download assets which were part of the base game, if changes were made to them. How, and how much depends on how the content is packaged and is delivered.
Since the Isle of Siptah building materials and armour will be usable in either of the two maps, you will most certainly have to download those art assets. However, those should only represent a small fraction of the total download size.
I’ve heard in several modder Discord channels, that significant changes were made to how certain assets (blueprints?) interact with the game. Such that, any mod ‘cooked’ with the previous devkit will not work at all.
For example, there is generally a testlive option to convert your base game into a testlive client, in exchange for significantly smaller download size. The size varies, but is usually around 4-5gb, to accommodate anything new or altered. This time, the download size was 60gb. Since there was no time saved, I cancelled it and downloaded the testlive client separately. I inferred from this, that the day 1 patch would be rather substantial.
I realize that this is frustrating. Take a breather and, If you live in an area with good infrastructure, run the download overnight. There are those who are much worse off, particularly in rural areas; where the only internet connection available is phone tethering. I’ve been there myself, and have had to spend months downloading games piecemeal.