Circuit design for triggering multiple optocouplers at constant voltage level
This will be easier if you add reference designators to all components.
2a) 150 mW (or 167 mW) is not a lot, although it would help to know the part number or at least the package type of the internal MOSFET to determine its power handling abilities.
1a) Look into an emitter follower circuit.  This will give a more stable output voltage.  Example:

Increase the 150 ohm resistor to 1K (to reduce the MOSFET power dissipation.

Add a small NPN transistor to the circuit, 2N4401, 3904, 2222, etc.  Base to the bottom of the 1K resistor, collector to the +5 V, emitter is now the new MARK_IN.

Increase the 1 K resistor on the MARK_IN line to 10 K.  This should be low enough to assure a logic 0 at the GPS input.  If not, return it to 1.0 K.

Add a 3.9 K resistor from the transistor base to GND.  With the 1.0 K, this forms a voltage divider that puts approx. 4 V on the base when the camera output is high.


After the transistor base-emitter junction voltage drop, the emitter (the new MARK_IN source) should be at around 3.4 V.  Because of the current gain of the transistor, the MARK-IN line will be much less susceptible to changes in load.