And changed from a comet to a planet in that span of time (around 3,000 to 4,000 years ago)? Did it transform while being ejected from Jupiter? Or halfway to its current orbit? Or all the way to its current orbit?
Planet-sized bodies with cometlike tails have been documented in exo-solar systems. It's a "thing". As ions, were ejected, the tail reduced to the current remnant tail that was documented by the Venus Express probe. Confusing differences in values of various tables, kept to record the visibility of Venus over the centuries, are consistent with an orbital track that was still arguably changing at 1500 AD. So yes, largely over the past 4000 years.
Where did it pick up ALL those orders of magnitude extra mass?HOW did it overcome the ejection velocity's tendency to shed mass? What made it stop where it is, especially after having acquired all that extra mass?
It didn't acquire more mass. The original body was planet-sized. It did shed most of its ion tail. The official astronomic view of Venus is that, by some process, it shed most of its water.
And, most importantly, if this can happen, why hasn't it happened again? Why aren't we observing it happen regularly with other comets within the outer solar system?
I'm not talking about a comet that transformed into a planet. I'm talking about a planet-sized body that resulted from a collision with a gas giant, and that exhibited a cometlike tail. And based on current exo-planet theories of formation, there's nothing whatsoever to preclude new collisions producing new astronomic bodies.