Why can Synology not offer a migration utility to go from Ext4 to Btrfs ... even if required to backup to offline media and then restore? Or, does something like that actually exist ... even if from 3rd party?
You can use Hyper Backup to do that, and it's all supported. In fact, that's their recommended path. But they don't have an in-place way to change the underlying format on the disk. When I moved from EXT4-to-BTRFS years ago, I took it as an opportunity to start fresh and build my system up from (mostly) scratch. It (mostly) paid off. :)