data analysis leads to linear regression model: how to proceed with prognosis?

Data analysis of a large dataset of project management data together with working hours led me to a surprisingly simple linear model over the key milestones of all projects. Now I am a bit at loss on how to proceed. The stakeholder wants a prediction on working hours spent per milestone and total working hours needed for one project.

1.) Do I calculate an average linear regression plus confidence interval and use that for prediction other project outcomes?

2.) Do I have to do something like a train-test-split for linear regression?

3.) Would bootstrapping sampling seem like a useful strategy to simulate more projects add any value? help me refine confidence interval and mean of slop and intercept?

Any discussion highly welcome!