Optimize my power button circuit to make it more robust
So my idea would be to add a new resistor with around 90k on the collector of T105. Together with changing R106 to 3k3 this will result (hopefully) in a better interference immunity for the switch (so I can´t turn on the device with my finger) and the same current consumption, ...

If the intention is to add R2 as shown in the following schematic...



simulate this circuit &ndash; Schematic created using CircuitLab
... then no, this won't work. Adding R2 is good but the values are not. Because, given that Vbat = 10.8V, VGS of T103 will be -0.4V (from simple voltage divider formula). It is below the threshold voltage (i.e. -1V), so T103 will never turn on. Besides, this modification will not help to prevent accidental turn-on by touching your hands.
You can try the following schematic instead:



simulate this circuit
With the modifications above, VGS-T103 will be 6.4V, so T103's turn-on is guaranteed.

Adding Css slows down the turn-on event of T103 and thus helps to reduce the inrush current. You can try with something between 1n and 10n to see how effective it is. Or you can tweak it according to your needs. Or you can remove it if not necessary.

Adding R3 in series with the S1 switch will prevent the turn-on-by-hand accident. If S1 is something like a momentary (non-latching) type push button then R3 can be decreased to something around 1-10k. If it's a latching type then leave R3 be 47k.