I just found myself implementing something similar for a different use-case and remembered this thread.
With AES-CTR, each block has a different counter, starting at the initial 16-byte value defined by the IV.
For a specific encryption key, each value of the counter should only be used once, i.e. an overlap in the counter ranges between different encryption runs is BAAAAAD. For a memo, we should be OK, since the size of a memo is just one AES-256 block.