Hmmm... Apparently I kind of solved it ☺ (but I'm still open for suggestions)
It appears that my expression for BRICK's Stroke Width property "didn't quite work" because I was putting a specific frame number in the brackets of valueAtTime expression, hoping to get the value of THICKNESS from that particular frame in timeline. After learnig that valueAtTime uses seconds instead of frames, I made some easy calculations:
my fps = 24
my target frame in time = 1419 1419 / 24 = 59.125
So: a 1419th frame is 59.125th second of the COMP
Below those two expressions (bad and good) for somebody who will need it maybe someday.
// THE OLD EXPRESSION (bad):
//THE NEW EXPRESSION (good):