If you want to use LUKS you have to give it always a full partition. LUKS encrypts on block device level. And that always deletes all the data on that block device.
The only other option is to use encryption per folder. Like with ecryptfs for example: https://wiki.archlinux.org/title/ECryptfs
I used this for a while because I was in the same position like you and I didnt want to give LUKS a full partition. But that was only for ca. 2 years. At the end of the day I realized that LUKS is the better option, I did a full backup and restored the data on a LUKS partition.