Back when I worked HVAC in the 90's I carried a army green flight line bag. I built it over 10 years using several philosophies.

1. You can't carry everything, and less is more.

2. Carry what you need to diagnose 90-95% of issues you expect to run into.

3. Carry what you need to fix 40-80% of problems. (depends on field and variety of tools needed), if you did 2# correctly you should know exactly what you need to go get to finish the job.

4. evaluate everything based on cost to carry and possibility of use. Example I carried couple dozen crimp connectors. I didn't need then often, BUT they also cost nothing to carry with no weight and fitting in a tiny side pocket.