No need to do this fully manually - the R4.0.4 installer can configure LUKS-encrypted btrfs:
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In the “INSTALLATION DESTINATION” screen, select “I will configure partitioning.” (If the destination disk isn’t empty already, you may also want to check “I would like to make additional space available.”)
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When the “MANUAL PARTITIONING” screen opens, switch the drop-down menu under “New mount points will use the following partitioning scheme:” from “LVM Thin Provisioning” to “Btrfs”, then press “Click here to create them automatically.”