How do i set a “world spawn” *per player?*

What I need is for different players to have their own “world spawns,” so that they can set their spawn differently with beds but if their spawn is destroyed it is always set back to their personal default spawn. Mind you, I want different players to have different personal default spawns. The issue comes as the spawns are not pre set. The player can decide where their default spawn is. I have thought about numerous ways of saving these positions to reset their spawn back, but I simply don’t know how to set their spawn equal to the variable saved of it because you cannot modify player NBTs and you cannot inject a variable into /spawnpoint. I could use an invisible entity that sits on the spawn, but if it falls out of render distance it will not work. If anybody has a solution to this perplexing issue I would be much obliged!