First of all, symlinks don't work on Android's emulated storage (
/sdcard). See this answer for details.
Secondly, when you format external SD card as Adoptable Storage, it's mounted at
/mnt/expand/[UUID], which holds a file hierarchy very similar to
/data partition, like
/data/media etc. If you migrate your data to newly created Adoptable Storage, data from
/data/media/ is moved to
/mnt/expand/[UUID]/media/ and the later is now emulated and bind mounted at
/storage/emulated. You can check this by executing
df on a terminal app or
adb shell. It should free space on internal memory i.e.
userdata partition mounted on
Additionally you can move apps (along with their data) to Adoptable Storage to further free up space. Apps only with
installLocation set to
auto can be moved.
So you can't create symlinks, but if you want more free space, go for e.g. replacing large directories in
/data (check with
du -h /data | sort -h) with (bind) mounts from partitions on external SD card. However (DAC and MAC) permissions should be taken care of, otherwise apps (or even OS) may break.
See Can't move apps from internal to external storage.
RELATED: What is /storage/emulated/0/?