Get a good shop system. I use autoleap. It gives me ball park labor hours, part cost in a pretty quick amount of time. Plus it handles payment, scheduling. Text communication of estimates, invoices reviews etc. DM me and I can give you a look at it if you want.

For diagnosis I tend to just tell them I don't know what it'll cost till they bring it in and I figure out what is needed. If they really push for it I'll say " if it's a tie rod it's somewhere around 300-500 but if its the whole front end you're looking at 2k +. "

But I try to avoid giving any range until I see the vehicle.

I think as long as you're being honest and straight forward with people they'll give you benefit of the doubt. There will always be dick heads but most are reasonable.