There are at least two ways to back up Memento that do not involve Memento's cloud.
In the global Memento menu on the phone, scroll to "Settings", then scroll to "Backup". This will create a zip file containinng all of your libraries. There is also a "Restore" option there if/when you need to restore that backup.
You can individually export the contents of your libraries to a comma delimited file. This type of export is handy if all you want to do is restore the contents of an existing library.
To me, I buy the subscription to support the developer. If the developer is not supported, the product could cease to be updated and may not any longer function. I have seen this happen in other applications on Android. The HanDBase application itself almost had this happen to it. I also like having the Memento cloud so that my important libraries are automatically backed up immediately. But even so, I will do a weekly backup of my entire library (as described above), and I also will export my most important libraries to CSV format at the same time. And it is not being paranoid when Murphy is involved!
BTW - That Memento export/import feature is what I believe to be the easiest way to convert from HanDBase to Memento.