First thing is use core wood poles as you'll double how high it can be. You'll also want to build from both sides most likely so you can meet in the middle with structural integrity. Also might need to raise the land a bit on 1 or both sides to make the gap smaller.

Need better support. Structural integrity goes down with every piece used that isn't connected to the ground. Different building blocks retain integrity are varying rates compared to each other. For example, wood beams lose integrity faster than log beams, which lose integrity faster than wood iron beams. If you want to make more impressive structures, you need to use more impressive materials.

Not really, just takes some time to figure out the structural integrity system. You can only stack the same type of thing so high before it starts to fail. If it is stable at that height, I would run the lower section all the way across the water and then start building from that side to meet in the middle, you should be able to span a couple sections without beams going all the way down to the ground.