Sorry to wade into this a bit late, but you’re quite right about the default LUKS sector size… seems sub-optimal.
However, Thin LVM chunk size will have a minimum size of 64KB, and is usually larger depending on the pool LV size at time of creation. My main system uses 64KB despite having a large pool size; I assume this enhances random write performance but haven’t tested it. #write_amplification
On the ‘cost’ of Thin LVM snapshots: Making snapshots is essentially no cost, but deleting (and oddly enough) renaming snapshots takes a significant amount of time. The latter are processed by the kernel in a single-threaded fashion and I usually see 80-100% CPU for >5s when Qubes or Wyng deletes a large snapshot.
Btrfs - My understanding is that it is extents-based but has a settable minimum sector size via mkfs.btrfs with a default of 4096. I think a good basis for comparison would have LUKS set to 4096, Btrfs at default 4096, and Thin LVM pool at 64KB.