Comment 6 for bug 1881932

You can just move to @ and @home manually after install.
My issue is a different one, Subiquity doesn't support installing on existing btrfs (I have RAID1 setup), it errors out with exit status 3.

My workaround was to
1. install the system on a separate drive
2. manually copy everything to new btrfs subvolumes /@ and /@home
3. make it bootable by booting from live ISO and running:

---
# be careful here, don't set the wrong device
DEVPATH=/dev/sda
DEVPARTPATH=/dev/sda2
MNTPATH=/mnt/sda2
MNTOPTS="-o subvol=@"

mkdir ${MNTPATH}
mount ${MNTOPTS} ${DEVPARTPATH} ${MNTPATH}
mount -t proc none ${MNTPATH}/proc
mount -o bind /dev ${MNTPATH}/dev
mount -o bind /sys ${MNTPATH}/sys

# get into the target system
DEVPATH=${DEVPATH} chroot ${MNTPATH} /bin/bash

# generate Grub2 config grub.cfg
grub-mkconfig

# generate Grub2 menu.lst file
update-grub2

update-initramfs -u -k all

# clean bootloader ; 512 (MBR) = 446 (bootloader) + 64 (part table) + 2 (signature)
dd if=/dev/zero of=$DEVPATH bs=446 count=1

# install Grub2 to MBR
grub-install --recheck $DEVPATH
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