What tack like options are stronger then blu tack?
I can't find any sort of moldable "adhesive putty" that isn't in the same basic class of product as poster tack. Products like rubber cement, which are generally removable from most surfaces without damage (not much anyway), stay pliable but not moldable after they cure, and almost anything else stronger than that should be considered permanent, as it will mark or damage the surfaces being adhered.

Beyond those products, pretty much everything you give as examples of things to hang on the wall non-permanently are what Command strips were designed for. Yes, they're one-use, but a couple of Command strips can hold some surprising loads.

Why does it need to remain moldable? Are you trying to find something you can pull out year after year along with the lights, and not have to find replacement adhesive strips for? If so you're probably out of luck.