First I’ll address the subdivide question. You can indeed subdivide, but if you’re going to cut, by using ‘Delete Hidden’ after you hid a selection, then you’d have ‘Delete Lower’ on your Subdivide menu. Of course after this you can’t go back on your subdivision levels. But in your case it doesn’t really matter.
Now, to have those sharp edges you must have a high poly count to simplify the process. Above 200k polygons should do the trick. You can check your poly count on the superior right corner. Like this you won’t find the spiky problem you mention.
For the mask, I’m using the insert brush, for the ‘breathing system of the robot’ (little circles and rectangles). And for the edges I guess I used just the DamStandardBrush. You can use lazy mouse or shift while dragging.
To be honest there’s at least 4 more ways to go around hard surface sculpting. But I guess that I’ll leave that to your imagination.
I’m still learning too, but I don’t want to give you more advantages LOL!
Check this series:
https://www.youtube.com/playlist?list=PLkzopwqcFevbD6QZnhzROSyiaFUCZxPz3
You should also look at any Zbrush tutorial to get more ideas, there’s plenty on YT.
Cheers.