The KDE Partition Manager is capable of resizing LUKS partitions. This is, in theory, completely safe. However, due to the nature of messing with partitions and LUKS especially, I recommend taking a backup beforehand, and running the operation from a live USB.
Naivly, I tried both options , hoping that either would work and I do not have to spent too much time on doing (learning) it manually …but looks like it’s difficult without.
… thus it would be great to have a no-fuss easy default install … what the GUI even does, just without a working option to leave some space for another (not encrypted) partition