Could there be a provision to have a portal for end customers (i.e customers of our agency clients) so they can manage bookings, payments, send documents and communicate with our clients.

vcita.com has a portal solution for agency clients. So we must also have something similar if not better.

Also could the portal be used for multi-purpose based on the client business/niche. i.e. a tenant portal for landlords or property businesses to manage tenants, payments, submitting issues, uploading & managing files, images, agreements and documents etc. Here's a sample screenshot.