To give finality to this thread, the conclusion based on my tests is that
61 is actually the real max length of the data part of the payload based on the header I presented above. Trying to send a 62 byte payload with
RF69_MAX_DATA_LEN=62 does not work.
If you want to implement your own encryption you can go up to 255 bytes. Also you may drop a header byte or two if you need 62 or 63 bytes.
For reference, from section
5.5.5.2. Rx Processing:In Fixed length mode the Message part of the payload that can be encrypted/decrypted can be 64 bytes long. If the
address filtering is enabled, the length of the payload should be at max 65 bytes in this case.
In Variable length mode the Max message size that can be encrypted/decrypted is also 64 bytes when address filtering is
disabled, else it is 48 bytes. Thus, including length byte, the length of the payload is max 65 or 50 bytes (the latter when
address filtering is enabled).