I’ve never done it with LUKS, so I’m not sure what issues you might encounter there. FWIW, I have done this sort of thing before with non-LUKS partitions: BTRFS partition corrupted - #22 by glb
No. It doesn’t matter with regard to your partition boundaries. Just don’t completely fill all your partitions with files. It doesn’t even matter exactly where on your SSD the unallocated space is. The “unused” space could, for example, be in your ESP.
Again, I haven’t done this with LUKS, but yes, there are (at least) two things you need to resize – first your partition, then your file system. Some automated tools may perform both those operations with “one click”, but I don’t know. I haven’t used the newer (more automated?) tools.
P.S. To let your SSD know about the free blocks in your ESP, your ESP will need to be formatted with VFAT to support trim/discard and you will need to either manually run fstrim ... on your ESP once in a while or add discard as an option for the ESP mount in your /etc/fstab.