>Falcón heavy is not human rated, and will never be.

It could be if the desire was there. SpaceX originally planned to do it when they had a customer who wanted to fly on it, but after the customer changed their mind SpaceX no longer had any reason to do it.

If NASA asked SpaceX to crew-rate Falcon Heavy, it would be done. It would hardly the first time SpaceX changed their minds in order to meet NASA's requirements.

Also, the fact that Falcon 9 is crew rated, currently flying on a regular basis, and arguably the most reliable launch vehicle in history means Falcon Heavy is starting from a good basis, moreso than SLS I'd argue.

>SLS is 80% more powerful

In terms of raw thrust, sure. But it's also very inefficient, such that in terms of payload capability it's only around 25% more capable to TLI, and most of that is from the high efficiency upper stage. If you put a similar high efficiency stage on top of Falcon Heavy (traditionally the SLS's ICPS is proposed, but Centaur V would be even better) it actually gets pretty damn close. With propellant crossfeed in the mix you'd all but match it.

Such developments would take time and money, but would still likely be cheaper than SLS in the long run. However much like with crew-rating SpaceX would prefer to focus on Starship, and NASA currently show no interest in developing such capabilities, though the previous administrator did raise the possibility.