I think you mean Priority Assignments, not Investgations; right?

Well the problem is that this part of the game is actually bound to the wh40k lore. You are radical or puritan. You can't be both and have to choose wisely and stick with it. Many people already had your problem (me too) but the only REAL solution is basically to start a new char.

It is actually possible to get back to 300 through investigations (I did it myself with my Psyker from 150 or something) but it takes several weeks of checking the possible investigations on a daily base...

I too fell short of 300 one way or the other.  Work thru Random Priority Assignments.   Some have a +15 Morality choice right up front, the last one I did to reach 300 had a +25 Morality choice a few steps into it.

If you have finished all campaigns/DLCs the only way is to look after investigations. There is a chance that there are morality decisions. But they give +15 only so it can be a looong way to max out your morality tree again...