Great work on starting a new venture. A great idea and very timely given the buoyancy of the property markets. An app would certainly help both improve your efficiency and streamline your processes especially if you looked to collaborate the full workflow i.e. invoicing, document storage, client portals etc. In terms of developing an app this falls outside of my property expertise, but I imagine there would be a number of development companies or even freelance developers that you could buy a coffee to get an indication of the cost, timeframe and capabilities of your app. If it were me I would seek out google or possible development pages on social media channels or chat forums to dig deeper into who may be able to help you here.
https://www.puttiapps.com/ could be a good starting place for you here.

All the best with this new exciting venture.

Cheers!