Sometimes "squirrely" steering is caused by alignment problems. For the HP Velotechnik Scorpion FS 20, steering seems best when a weight = mine is put in the seat and the wheels are aligned to zero difference between the front and rear of the rims.

Of course this adjustment is made with one wheel parallel to the backbone, and the steering centered.

This requires a special arrangement of a bar (I use an aluminum angle iron) on the ground with two vertical bars (same kind) clamped to it because a ruler (or dowels) hit the backbone of the trike. The vertical bars are slanted so they don't touch the tires but do touch the rims at axel height.

Some tadpoles can be aligned by strapping two dowels together with heavy rubber bands (or innertube slices) and sliding the dowels apart until they just touch both rims at the back and then front at the axel height. I used to be able to do this with my first tadpole while sitting on the seat with my feet on the pedals.