I don't think that's a better solution and it is a misuse of linestrings. Again, the reason why I've deleted the forests is because not everything inside the national parks are forests, and there are substantial patches of wetland and grassland inside of national parks. Also, there are some existing mapstyles that use "green" as the boundary of the national parks, not forests, such as OSM americana (https://zelonewolf.github.io/openstreetmap-americana/). There is little benefit from keeping these relations and I urge you to delete them.