There are ways to create working hard links, but that would only work file to file, not directory to directory.
You can create symlinks pretty much anywhere else in /data but /sdcard (in all of its forms) won't work. You could create the symlinks in /data/media/0 but, although an entry for them would appear under sdcard, it would be inaccessible. The only way to successfully access those symlinks would be via /data/media/0
The reasons for this (in extremely simplified terms) is that /sdcard is emulating a fat32 file system, even though the underlying system is ext4 or f2fs (depending on device/ROM). fat32 doesn't support symlinks.
If you want some more background see https://www.xda-developers.com/divin...e-io-overhead/