Please help with trebuchet: counterweight=1.5kg, projectile=0.04kg, max CW height=0.8m, pivotpoint height=0.6m

You don't give any other restrictions, and don't say what the performance criteria are. Having the CW *slightly* higher than the axle is slightly unusual, but normal with fixed-counterweight trebuchets and old-style floating arm trebs (the two-wheel variety, not the 3 wheelers). A whipper with a low beam ratio and a shallow ing angle would also fit, and would give the best performance of the three choices. The fixed CW type would be easiest to build, but would give the least performance.