For getting closer to the nearest higher ground, I think it would be helpful if there was a way to consider unranked peaks. For example, Monroe County HP in Florida (elev. 17’) shows an isolation of 565.87 miles to the nearest ranked peak, Graveyard Hill in Alabama, but Virginia Key (elev. 32’) is 60.79 miles away, some 500 miles closer. Of course, this does not identify the absolute nearest higher ground, but it at least narrows down the possible distance.
I don’t know what the difficulty of programming this would be, so it is just a suggestion.