you probably already know this, but for anyone else:

The Cosmere Series (of which the Mistborn Saga is a part of) does heavily feature Sci-Fi as well as post-apocalypse themes alongside (mostly) fantasy (Sci-Fi: the sunlit man, tress of the emerald sea; post-apocalypse: Stormlight Archives, Yumi And The Nightmare Painter), which made me think OP was talking about this series specifically.

In some of the other books it is mentioned that all of the powers originally came from a being called Adonalsium (basically God). what fuels all these manifestations of powers is called Investiture. Each Shard of Adonalsium manifests different Powers, Allomancy is just one of them.

so it’s a unique mix of classic fantasy, sci-fi, and post-apocalypse genres in a single gigantic saga, in which the sci-fi and post-apocalypse themes are intentionally kept vague and in the background.

highly recommend all of the other books!

they are great in their own right, and also give a LOT of extra bits and peaces of the overall lore!

what’s best about the series is, as you’ve already explained, the “hard-fantasy/sci-fi” approach to powers: all power requires some kind of source, everything comes from something.

best to do the Stormlight Archives after Mistborn (either order works), then the rest; order doesn’t really matter, although i recommend Tress of the emerald Sea and The Sunlit Man to be read last, because they contain a lot of sci-fi lore, which is best enjoyed last (imho)

also: Stormlight Archives Book 5 is coming relatively soon, i think it’s december?