Some additional remarks:
There is a bug in the BC817 model file posted in Post 4. KiCad/ngspice complains
**Error in line inc. bjts**
**Not enough parameters**
Why not take this message seriously? The model file contains a line Inc. BJTs. What is that? If a line starts with I, ngspice looks for a current source, with parameters like 2 nodes and a value. This is not there. Well I know that this is a mis-placed line break, and Inc. BJTs belongs to the end of the line above. Unfortunately such things are strictly forbidden, as they always will lead to mis-interpretation and error.
Using global labels for input and output is o.k., if we are doing simulation only. A global label eases readability, and you want to plot out versus in.
Of course you can (with some effort though).
Please have a look at Generate chirp signal in Kicad .
Another way may be using the function generator ICL8038, see Simulation examples for KiCad8/KiCad9/Eeschema/ngspice