Ok well, i didn't completely solve my problem but i've made some progress so i post here because it might be interesting to Ampa (and maybe others).
I found a cleaner way to change the pen pressure (even if i still need to open Wacom properties and get to right tab for the script to work) :
SendMessage 0x405, 0x00000001, 0xffff800?, , ahk_id 0x90b66
...where
? is the position on the slider (from 1 to 7).
0x405 is the type of message i send, TBM_SETPOS, which is a message specific to sliders to (yes) set its position.
0x00000001 is the lParam value, which i found using Winspector.
0xffff800
? is the wParam value.
0x90b66 is the HWND of the "Pen pressure" slider.
I hope these values will work on other systems.
I'll keep trying to improve this

EDIT> ok, looks like i got enthousiast too fast. It changes the cursor's position but it doesn't actually change the pen pressure. It's like moving the cursor without "validating" its position. So it doesn't work

EDIT AGAIN> also, i was confused about HWND, i didn't realized it wasn't permanent, so of course my script was all wrong.
Now i use this and it works fine even if it's not really as clean as i'd like it to be :
Pressure at 7 (max) :
Controlsend, msctls_trackbar322,{End}, Propriétés de la tablette Wacom
Pressure at 5 :
Controlsend, msctls_trackbar322,{End}{Left}{Left}, Propriétés de la tablette Wacom
Of course here i use the french window title "Propriétés de la tablette Wacom".