Hi Martin. This is possible with the Softr Client Portal template plus a few tweaks to the template.
The trickiest part is managing the user permissions or "User Groups" in Softr. How you'd do this is create a single Softr app with the Client Portal template. Then you'd manually create User Groups for each of your clients.
e.g.
Client - Company A
Client - Company B
Etc.
You'd also have and admin User Group for each of your clients.
E.g.
Admin Client - Company A
Admin Client - Company B
Then you can manually assign your client users with the admin user groups who have permissions to blocks that only they have access to, whether it's uploading documents or viewing documents, etc.
Then each of your clients' clients (end users) would be assigned the User Group of their vendors e.g. "Client - Company A"
Where Company A is your client and is also your client's clients' vendor.
Then that User Group can be used to control what documents can be seen or uploaded by your clients' clients' (end users).
These User Groups can be assigned manually by you for every admin or end user on your Softr app's "Users" page under the "User Groups" tab.
However, I believe there is also a way to automatically assign User Groups to users on sign up of end users, though I've never tried this. It involves using a Softr Form block, Make webhook to the trigger a Make scenario which uses the "Create a User" module which creates a user and assigns a User Group based on a hidden field.
Then your clients can add your app to their website either as a link or iframe.
So to answer your question, yes this is manageable and scalable. Whether it's a waste of time or not depends on how important this functionality and user experience is for your clients and their clients.