I'm not quite sure if I understand correctly what you can do already and what not. But I'll try to write you some of my experiences with the front crunch.
It took me a long time to get Mucki to do a proper front crunch. What I got early and quite easy was a deep stretch between the forelegs when he was standing with them on a pedestal. Maybe that's what you experience with the pile of logs you mentioned?
That stretch though was not at all mentally linked for Mucki with the image of the front crunch that I wanted. Mucki is also not a horse who does the front crunch naturally very much or very accentuated, so I assume it's depending a lot on the type of horse as well.
I started to park out the front legs and then tried to lower his head. That worked so and so, but mostly ended up in leaning too much on one foreleg, so that when I asked for a backwards movement, we ended up in a bow.
I let the front crunch rest for a long time and only recently came back to it with a new strategy. I used my hand target to guide his head straight down from a normal, square standing position. I rewarded him for following my hand further and further down without shifting the weight to either leg, but keeping a nice, equal weight balance on the front legs. That was the most important thing for us!
It is much harder to stretch down without bending one leg, so as soon as he understood that he was supposed to keep his legs straight, I could guide his head backward with my hand-target and he would keep the legs straight and firm, but park out the hindlegs a bit to compensate and then take on more weight on the hindlegs.
The trick was for us, not to touch the breast with my hand as this would get him into jambette-mode pretty quick. Instead I switched to the other hand which I presented him as target from behind and between his front legs.
I hope my description doesn't confuse you any further - it's not so easy for me to describe these things precisely