To make the intellisense and the build work I suggest using the project path as a base. The project path is always set as a compiler include path and for intellisense the same applies to shared projects.

This means that at any point in your code you could use:-

#include "src/SharedTest1/myClass1.h"


Building Arduino has to be performed in a hidden temp folder so that the .ino code can be combined to cpp. That does add a layer or complexity v's a standard c++ shared project. The solution above will work but if you can think if any other "include" paths we could add just for intellisense then please let us know. It would be nicer if intellisense would be same as compile which, as you say, allows better relative #include "SharedTest1/myClass1.h" syntax.