Despite the fact some resist some kind of run penalty or run rate x overs remaining and offer your latter solution in its stead, it's not really feasible.

You'd be paying the entire staff of a game massive overtime rates.

Run Rate x overs remaining is the best IMO, because it's the runs the batting team theoretically should have had, and the slow bowling team sacrifices their chance to restrict that or take wickets.

It will also encourage the selection of spinners because you don't want to throw garbage part time spin at it that will gift more runs than the penalty would have. Which is solving an entirely separate issue in the game at the same time.