There was one more solution employed in various places: aftermarket additives. These could be actual leaded additives, or more likely some substitute which would somewhat reduce wear on valve seats. These additives were not commonly used in cars in the US, but were/are commonly used in other countries where leaded gas disappeared faster than old cars, as well as in the US itself in farm and garden tractors, whose engines often last multiple decades.

Perhaps we could do the same for small planes, but clearly it would take a lot of research and testing to make sure it's safe. If someone is willing to pay for that testing, it's probably doable.