I think the question here is whether DeDRM will work on current ADE 4.x and GooglePlay books on macOS Catalina or Big Sur. If I'm understanding correctly, dynabook would like confirmation of this before taking the plunge.
Also, it is still possible to upgrade to Catalina. Info from Apple:
https://support.apple.com/en-us/HT211683
Direct link to Catalina installer:
https://itunes.apple.com/us/app/maco...314?ls=1&mt=12