i want to limit the current through a MOSFET (high side) as simple as possible. The MOSFET will switch 600V and approx. 150mA for 1-2 seconds. The focus really lies on simplicity. I know that i can control the drain-souce current through gate voltage. But is this reliable?
Here is my basic idea. Please note that I don't normally design high voltage or high power electronics. And I am not even sure the OP is using PMOS. So just let this be a point of discussion rather than an endorsed solution. Note that dissipation in R4 needs to be considered. Also, @Autistic made that comment about hot-spotting.
You might not be able to source a p-channel MOSFET easily. You can invert that whole circuit and use a n-channel FET (and an NPN). The calculations would be approximately the same.
Note that at 600 V and 150 mA for 1-2 s, that's a power of 90 W, or about 135 J. Be sure your FET can handle that.
600 V, 150 mA 1-2 s corresponds to a capacitor of C=I.T/V = 375 uF. Is that correct ?