In the live session, open a terminal and run:
sudo nano /etc/calamares/modules/partition.conf
Use the arrow keys to go this line:
defaultFileSystemType: "ext4"
Change ext4 to btrfs:
defaultFileSystemType: "btrfs"
When done, press in turn Ctrl+o, enter and Ctrl+x.
Next launch the installer and proceed as you did before. Hopefully this will work out the way you want.