I have a code reader, so I'm good there. And most external, and some internal, parts replacement I have the knowledge and tools to do.
Where I will take it to the shop is for something I might be capable of doing, but I don't have the time/expertise to get it done as quickly or without pain. There are things that you can do in the car, but taking the engine/trans/etc out would make it 5x quicker and easier.
Example of something I'm working on now. My wife got an older Mini (R50 Cooper). And it runs great, lower miles, just need some stuff because it's older. Already did the shocks, brakes, had to replace the oil pan, now I'm doing the ball joints (inner and outer), tie rods and bushings (including control arms). I got the sub frame out today, even though one of the nuts on a tie rod end didn't want to come off. Going to get everything cleaned up (old pan was leaking a bit, so sub frame is dirty as s***), put new bushings/ball joints/tie rods on tomorrow and get it put back together. All told, about $800 in parts. If I had a shop do all of it, would probably total about $4000.
Of course, it helps that I have all the required tools, those would be another $3-400 or more.