Btrfs lets you set per directory raid-1 even after the filesystem is mounted. Please read the documentation carefully and maybe even practice in a virtual machine with big+small virtual disks.
Discussion
VM is a good idea. Part of why I haven’t run a lightning node is me not being good at data redundancy/real-time backup…I thought I was clever with raid1, right up until I had a discrepancy and no way to know which was right automagically…