Sounds like it might be formatted (used) as an expansion of internal memory. That’s known to cause all sorts of problems and is not advisable. The trouble is, people don’t know about the problem until they encounter it and by then it’s often too late. Have a look here.
The recommended usage is “Portable” (sometimes also called “external” as opposed to “internal expansion”). That option has drawbacks too, in that the data are not encrypted, and the card can only be used for user files (photos, documents, videos, audio) - but those are usually what uses up space on the phone anyway.
Give it a bit more time. How big is the SD card? If it is formatted as internal expansion then it’s not surprising that the update is taking a good while.
P.S. - DON’T try and change the SD card formatting or remove it for the moment. That will pull the carpet from under Android’s feet and you’ll almost certainly lose data. Just wait for the moment.