I've published a tutorial on using Backendless for your Dropsource app API: https://help.dropsource.com/docs/documentation/integrating-with-dropsource/api-tools/connect-your-app-to-backendless/
It covers setting up your data in Backendless, creating your Swagger/OpenAPI specification, and connecting to it in your Dropsource project - to log users in, retrieve user data, and post user data from the app.
I found Backendless really quick to set up, it has the advantage of automatically exposing REST API endpoints for your data tables, so you don't need to actually create the API, all you do is define your data model and it does the rest. One drawback when comparing to Bubble is that Backendless doesn't export a Swagger spec from the default setup (it does if you use custom business logic), so you need to author your spec to use it in Dropsource, however I've included a sample that should hopefully help get our users started, the tutorial also outlines what steps to take in order to create your own.
I'd say the process took about the same time in Bubble and Backendless so they're both great options for use with Dropsource, which one suits you best will come down to your own use case, skill set, and preferences..
Hope this helps someone!