I think there are several points to take into account:
- Format: you want the issue contents to be readable, this involves the choice archiving, compression, and encyption algorithms, using formats that are likely to be usable in the future.
- Encryption keys: similar to the above
- Use case: why/when you expect to use the stored data
I think we should focus on this last point.
If the data being stored was public (for example, the kind of content that might want to preserve a library or museum) there would be no point in encrypting it, the safe would be just to protect the medium (such as from a fire), whereas if there is some kind of confidential data both the safe and the encryption would be useful to prevent it.
Assuming these are indeed confidential (say, your business backups), we may want to focus on which cases you may want to access the information. Is it for some legal reasons, in order to fetch that deleted data two years after or to recover all your data after a catastrophic failure of your systems (e.g. a ransomware infecting the whole company)?
I think the company should have some kind of secure storage -such as a password manager- for the multiple keys it will need to handle (computer systems, bank accounts, domain registrars...). Adding an entry for the backup encryption would be trivial.
However, if we expect to recover from a complete IT failure using these backups, the encryption key should also be available somewhere else (and the safe pin, while we are at it). It is unlikely that in such case you needed the backups from 30 years ago, though.
In fact, it is quite unlikely that you suddenly need a file 30 years later. The only case I can think of would be for some historic digging.
As for the problem at hand, I think it may be a good idea to encrypt it, just because it is simple to do that on creation. If you want then not to be encrypted, you can simply store the key along the media in the safe. If you want to protect it better (say, you got a second safe at a different site, or at the bank), you move the documents holding the keys there. I would not advocate for only having the keys there. I would still keep them at the password manager, and perhaps on every backup (or every year supposing you don't cycle the keys for each backup), print a whole list of keys from there to store (thus you have a recently printed copy of the old encryption keys, thus thwarting that the encryption key was unreadable due to age).
A different approach might be to use a weak password on purpose which is so easy and memorable enough so that you are sure it won't be forgotten on 30 years, and even if it was, that you would be able to reach someone that knew it and still remembered it. It's still a gamble to bet that it would be remembered, although you might want to combine it with other approaches as a last-layer (assuming that a relatively known key would still protect them appropriately for your needs e.g. from the cleaning crew).