Odd. There doesn't seem to be any widely used library or framework for writing encrypted chunks to an append-only file. No standard format. We could really use a taxonomy of encrypted-chunk schemes.
There are some heavyweight event logging suites that can write encrypted log files, but I don't see anything for simply writing arbitrary data. Is there a keyword I'm missing?
https://old.reddit.com/r/cryptography/comments/1ls4n07/how_to_approach_encrypting_appends_to_a_file/
Some encrypted archive formats (7z, zip?) allow appending encrypted chunks, but I haven't looked at the details in a couple of decades.