Unfortunately, you’re not going to be able to do steam leaderboards in any decent capacity using blueprint OR UE4’s OnlineSubsystem - you have to go around it using native steam methods in C++
The reason is - using UE4’s OnlineSubsystem, there is no way to read an arbitrary range of scores (like the top 10) - you can only give it specific player IDs and see those scores. Also, UE4’s OnlineSubsystem is just riddled with issues for Leaderboards…
I found this article helped a lot for accessing native steamworks functions:
From here, I wrote some blueprint-callable methods using the “Leaderboards: How-To” article under the steam partner site. (It’s pretty much copy-paste)