Hi There,
Firstly, whilst playing with this I noticed a bug that you might encounter for a short period - note that a permanent fix is on the way for that Brush tool closes automatically in psychojs/online, despite closeShape : false
To save data though, can I recommend you add a mouse component that saves data on every frame. Here is a demo file 
https://gitlab.pavlovia.org/lpxrh6/basic-brush/tree/master
Hope this helps,
Becca