Almost none of this is intuitive without hundreds of years of hindsight. The more subtle aspects of stability, such as avoiding oscillation mostly had to be determined experimentally. Then there is also the matter of actually constructing a plane, if you want it to be useful, it's going to need to be a lot more then some folded paper.

Thrust was definitely also a problem, a glider is not particularly useful unless it has a huge lift-to-drag ratio, which is only possible with modern materials and a solid understanding of airfoil design, which is a whole other can of worms.

Even things that seem so basic that we don't even think about them, like high were not at all obvious: just look at Sir George Cayley's gliders.