Illā and siwā evoke exception, whereas lākin introduces a corrective or contrastive turn. In emphatic praise, these constructions can promise a qualification only to supply another excellence.
After “he is the moon,” saying “except that he is a sea” does not cancel the previous comparison. It shifts the image to a new dimension of praise; the following *lākin* does the same.
These connectors need not belong to a single grammatical category. What unites them here is the rhetorical reversal of expectation.