> But we can't even make a sustainable replacement for it that wouldn't require 4 kilograms of external batteries that you'd need to recharge multiple times a day.

There's no magical solution.

The heart develops several watts of mechanical power 24/7 [1], i.e. that's what is required even if the pump was perfectly efficient, and I doubt pumps are that efficient.

[1] https://hypertextbook.com/facts/2003/IradaMuslumova.shtml