I have been attempting to use a datapack to modify the valid_spawn tag so that I can force the player to spawn on different more interesting blocks, but as of yet I have not found any method that will actually work.  Before when datapacks were loaded after the world spawn was already set it made sense that changing the tag did nothing, but now it should be possible to change this tag in order to change how the game searches for valid spawn locations, such as allowing us to spawn on sand again like in older versions.