Mobile App development for Residential house building contractor

I want to develop and app that can be accessed by my sub-contractors to make scheduling and management easier.

The app would basically be a simple GANTT chart as the dashboard showing all current jobs and information as shown on the attached PDF shot of my Excel spreadsheet I use to run jobs.

Basically I want to automate the GANTT chart with the following functionality:

Add Jobs (takes us to Job details where we can enter all relevant job details)
Add sub jobs (a sub job is a maintenance job of a normal job ie a call back to fix something)
Assign staff (who will receive a notification and will have to accept)
Automatically send job details through to staff (drawings, WHS documents)
Staff can add sub jobs, attached photos, adjust dates on the job
Staff can see a list of their jobs and access all details

Any suggestions with functionality are welcome. I have researched Apps like Tradify, Smartsheet, SIMPro etc and they are great but just a bit too much involved as I dont really require financial or job reporting etc

My main concern is the APP is very user friendly and easy to use as the end users arent generally technically sound.