My main solution is using code from a utility class library, that I wrote myself, but is a part from another solution. How can I set a breakpoint in the referenced DLL file?
This is not my own answer, it was Frep D-Oronge's suggestion in one of the comments above. It is easy and works with no hiccups:
"I find easy - just run two instances of Studio side by side. Ctrl-F5 on the 'primary' one to launch without the debugger attached, then attach to the process with the instance of studio that is editing the library project"
All credits are due to him.