Japan is an industrial country.
Is 国 pronounced as こく or くに?
(I was thinking that 国 could either be a compound word along with 産業, or it could be a noun which is described by 産業。Although, I think if there’s no の in between the adjective and noun, then it will usually be considered a compound word, and thus pronounced with 音読み？)
産業国 = さんぎょうこく = 音読み + 音読み
The same pattern is applied to:
農業国（のうぎょうこく; agricultural country）、独立国（どくりつこく; independent country）、先進国（せんしんこく; developed country）、国連加盟国（こくれんかめいこく; member states of the United Nations）、制裁対象国（せいさいたいしょうこく; a country subject to sanctions）、友好国（ゆうこうこく; a country with good relations with us）
Please note, however, that 産業国 is not a common phrase. It should be 工業国（こうぎょうこく）or 産業立国（さんぎょうりっこく）for industrialized countries. 産業大国（さんぎょうたいこく）is the alternative for the original sentence because 大国 means one of the economically large countries, and Japan is (declining yet still) the one.