Do not assign the result from “New Slot Instance” to the reference of your slot asset.
Just use the node when needed. Or you can just use “Save/Load Game Mode” nodes to save data from all your game mode blueprints.
“Save/Load Level” is for streamed sub-levels, do not use those functions to save the current persistent level.
Use “Save/Load World” or the “Save/Load Game Mode” ones, you don’t have to create new slot instances to use those nodes.
Yes you can have only one slot to save your data or many per level.
Most devs create a fixed list, 10 slots, where player can pick one to save.
“SaveGame” tag is necessary for any plugin function, if no tag then property will never save.