[atoav]

Just scale it down to 1080px height and save it as a jpeg with 75% quality. Nobody will ever see the difference. Most browser windows will never display that picture bigger anyways (that means you can use computationally more expensive downscaling offline vs realtime downscaling by the browser).

I wouldn't vectorize it, that would have you lose the hand-drawn feel of it.

[Escher21]

Okay, I've scaled it down to 1080 pixels in height as you suggested which brings it down to 1.5MB. I had to preserve the PNG format in order to maintain transparency.