Now that I am closer to understanding your situation on the ground, is there any room to move farther downslope to some flatter ground? If you moved to just beyond the end of the slope break, then more of your dam height as allowed by the regulations would be "effective" dam height.

If you are building the pond near the head of a valley, what will be your water source? (However, is your setting in a rain forest? Your flora look awesome.)

If you are using a pond liner, why can't you go nearly vertical for the pond side slope of the dam? That would help your geometry considerably. Just spoil your excavated material downstream of the pond liner location.

Do you have any low-growing vegetation that you like for the area that has a thick network of roots. Most of the dam design rules of thumb we utilize are for much wider dams than your project. You might be able to go as steep as a 2:1 slope on the backside of the dam if you immediately stabilized the soil fill with a solid plant cover so there was no chance of your backside material slumping down the slope. That would further help your geometry problem and gain a little more "effective" height for your dam.

How are you excavating the pond basin itself? The equipment you use in a tight space might also change your design parameters a little.