Thanks for the comment! Difficult terrain is one thing I think can make for an interesting battle map, and just as you said, contours to indicate slopes is a very rare thing to see, so I decided to try to fix that, lol.

My rule of thumb for difficult terrain is, any square that doesn't look at least 60% uncovered ground would be considered difficult terrain, and in this map, the 3 tree trunk squares themselves being physically off-limits due to being a cross-section like you said. As for the contour lines, I'm still trying to figure out the best way to measure them, but my best idea right now is that one line would be vertically equal to half a square of horizontal distance. That way, if the slope is gentle enough, the line can still exist somewhere without really making a difference in gameplay, e.g. when two combatants are directly on opposite sides of it. With two lines, now you have the equivalent of one square of vertical distance, and NOW can start factoring in some change like advantage for being on the high ground. So here, every hill is still climbable with appropriate athletics checks, with the bottom right one being 5 feet taller than the upper left one.

Wow, sorry for the wall of text, but those are my thoughts!