This is going to sound really weird, but I was able to create a decent trackpad signature once I got into the right mental state.

I held my pointer finger between my thumb and middle finger, and made myself really think of it as a pencil. I looked down at the trackpad as I wrote (rather than at the screen), and tried to visualize the trail it would leave on the surface.

https://seedlegals.com/ asked me to use my mouse for an online signature. In their FAQ they say I can't upload an existing one, quote "This is for security reasons, so that we know it is really you signing. [...], you might find it easier to perfect that sign on your phone / tablet with a touchscreen." I borrowed an iPad+Pencil from a friend and signed it with the pen, that kind of worked.