Support

As far as I can tell they do not offer an API for third party developers to send and manage files to Zoho Workdrive. Without an API we can't create an integration for it.

I'm afraid it's still not possible. It looks like you can only upload specific types of files: documents, spreadsheets and presentations. Look at their /create API service. The service and type parameters make it clear you can only upload these types of productivity application documents, either as standalone documents or as templates. You cannot upload arbitrary files. You can download any file, including images, videos and music. But, still, the problem remains that you can neither upload a backup archive nor does it seem to be a file type accepted by this service.

Even if that wasn't a problem, there are two more file size related problems. The maximum file size is 100MB (or 250MB if you're a paying user). Moreover, they do not support chunked uploads. Practically, even without file type restrictions, you'd have to use a part size for split archives around 10MB to upload anything to Zoho. Even a small Joomla site would require 2 to 3 parts. A medium sized site, like ours, would need tens of parts. This makes managing the backups impractical for you.

I think your clients are better off using BackBlaze B2. It costs $0.005 per GB per month. A site that backs up to 600MB with daily backups kept for 30 days with one backup per month preserved for a full year would cost you $1.155 per year on B2. I think that between the Zoho constraints, the integration with it being unlikely to work and the ridiculously cheap alternatives it makes no business sense for you or for us to pursue the integration with Zoho.