That looks really nice. I don’t make a lot of use of blueprints, but some kind of variant type like this is something that I really feel is missing. I actually made one myself for a marketplace submission I have, but it’s implemented using a statically typed variant so while it’s lightweight, it’s also more restricted than yours.
You should look into the experimental plugin in the engine called StructBox - it’s a blueprint wrapper to hold any struct, so using that you could extend your system to allow existing struct types as well. Not sure how stable it is though.
By the way I think you have a bug in UActorSet, you don’t appear to update the index after swapping on removal.