I suppose your 2nd use case makes sense if you possess the manpower and knowledge to maintain such a CRM/Project Mgt app.
For those who don’t, the google suite or some other off the shelf app might work for collaborative work, but maybe for what you guys do you need bespoke functionalities.
That said, you could also sell the app you build to other interior design companies as a SaaS which is usually a cheap and scalable preposition for the client. I’m sure you’re not the only interior company facing the comms issue with clients. That way, you benefit twice from building it!