I wanted to change the icons on Big Sur and ran into the same problems. I tried to follow the steps here (https://apple.stackexchange.com/a/395509/387852) but on step 6:
sudo mount -o nobrowse -t apfs DISK_PATH MOUNT_PATH
I ran into this issue
mount_apfs: volume could not be mounted: Resource busy mount: /Users/timrupp/newicons failed with 75
I really am no programmer or have any real clue how to use Terminal. I'd appreciate any help.