(define cryptnvme
  (mapped-device
    (source (uuid "uuid..."))
    (target "cryptnvme")
    (type luks-device-mapping)))

(define cryptsd
  (mapped-device
    (source (uuid "uuid..."))
    (target "cryptsd")
    (type luks-device-mapping)))

(define fs-root
  (file-system
    (mount-point "/")
    (type "btrfs")
    (device (file-system-label "btrfs"))
    (options (btrfs-mount-options "btrfs/guixsd"))
    (needed-for-boot? #t)
    (dependencies `(,cryptnvme))))

(operating-system
...
  (mapped-devices (list cryptnvme cryptsd))
  (file-systems
    (cons*
      ...
      fs-root
      %base-file-systems))
...
)