Shrinking encrypted ext4 partition

I have a micro SD card adopted by an Android phone as internal storage extension. While being adopted it was partitioned/formatted by the phone with two ext4 partitions: android_meta and android_expand, which is easily decryptable in a Linux laptop using the key under /data/misc/vold. I'd like to free up some space on the card for an "external" aka "portable" partition for music storage etc, ideally using that Linux laptop: the idea is to shrink android_expand somehow. How do I do that? E.g. GParted won't let me do it - if that's just 'cos it's encrypted can I perhaps decrypt it, write it back onto the card, move/resize it as necessary using GParted or the like and then encrypt and write it back again? Thanks in advance!