Indeed, I did ask for (and receive) help at
the GitHub DeDRM site. As recommended there, I found it easier to use ADE on my PC than to sideload RMSDK onto my Onyx Boox. Once I had authorized ADE with the correct account and removed the DRMed books from both the Boox eReader and Calibre's library on my PC, Calibre was able to unencrypt them when I reloaded them into the library and then transmit them to the eReader in unencrypted form.