The following simplified package definition ‘worked’ for me:

(define-public openjdk18
  (package
    (inherit openjdk17)
    (name "openjdk")
    (version "18")
    (source (origin
              (method git-fetch)
              (uri (git-reference
                    (url "https://github.com/openjdk/jdk18u")
                    (commit (string-append "jdk-" version "-ga"))))
              (file-name (git-file-name name version))
              (sha256
               (base32
                "1bv6bdhkmwvn10l0xy8yi9xibds640hs5zsvx0jp7wrxa3qw4qy8"))))
    (native-inputs
      (modify-inputs (package-native-inputs openjdk17)
        (replace "openjdk16:jdk" openjdk17)))
    [home-page ...]))

‘worked’, because it took to long for me so I interrupted the build in
the build phase.

About the labels issue: maybe "icedtea-8" could be changed to "icedtea",
"openjdkN" to "openjdk" and "openjdkN:jdk" to "openjdk:jdk" at some point
in the future, such that the package name matches the input label and
the input alist can be changed to a package list.

Greetings,
Maxime.