Can an independent shop replace the Torque Converter? or is there anything proprietary that only the dealership can do (e.g. reflash software that controls the transmission)?

So I got a quote last night to replace Torque Converter at $5k, and $12k for this and a new transmission. I think my current transmission is good, so I am only considering the TC

This is an expensive replacement. Should I go to an independent shop or AAMCO and ask them for a price to replace Torque Converter? How much is too much to spend on a vehicle that has 227k miles? The car overall is solid and the engine is in good shape, and looks OK. I’ve got a daughter just out of college who needs a car, so my hope was I could give this to help her get started so she can avoid a car payment.

How many miles? Does the AT light come on after awhile? And the check engine light?

The torque converter is supposed to act like a clutch so that when the car is stopped the transmission isn’t driving the wheels.

I’m not a mechanic but that’s my layman’s understanding of what’s going on. So no I don’t think it’s normal,