cryptography, for example, which is essentially math + cs together

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Which journal?

look at essentially any proceedings of any conference (in crypto we dont really do journals). see EUROCRYPT for example https://link.springer.com/book/10.1007/978-3-031-91098-2 in there, every paper will be cut down and referring to full version for proofs etc. which are typically on eprint.iacr.org

Well, yes, conference proceedings are usually page limited, but that's not a journal.

In AI, conferences are far more prestigious than journals.

We usually do conferences in cryptography/security, and most of them have page limits: CCS, USENIX, NDSS, S&P, CRYPTO, EUROCRYPT all have page limits (some allow appendices, which reviewers are not obligated to read).