Does look like you are bow riding. There is no point in trying to counteract ventilating by trimming motor in. You have to achieve good performance in neutral trim. Moving forward your motor creates a force produced by low pressure in front of blades and higher pressure behind. When you trim in direction of that force changes somewhat upward.
You can separate it into horizontal and vertical part. Horizontal moves it forward but vertical actually lifts your aft up forcing bow down over center of all forces acting on boat. What's more there would be low pressure area close to transome. Since you basically suck water from under boat. That would make boat stick more.
You are ventilating (are you actually ventilating or/and over-revving?) at neutral IMHO because of insufficient back pressure. I believe you should be able to cure it by using higher pitch and/or 4 blade prop. What is you current pitch? Do you have a tachometer?
Of course dropping engine a bit might help as well.
40 hp recommended rating is not to for purpose of aft load. That would be stupid. Lighter motor should plane lighter boat just fine. Rather because rated load for you boat is north of 2000lb and to plane that (or at least maybe like 60-70%) you need some power.