Thank you, SharpeXB , for the information about the HOTAS category. I found and tested controls of this kind for A10C and AV8B (I noticed them before but was unaware of the builtin behavior).
I tried TARGET for TWCS and the function
MapKey(&Joystick, TG1, TEMPO('x', 'y')); //short press X, long press Y
but in effect TARGET is too time consuming and too cumbersome to use for me. For the F15 I will keep the key-and-modifier solution and throw TARGET away.