I did find a solution!
It is (currently) 100% impossible to get the top scores using the OnlineSubsystem setup in UE4 (wasted 2 weeks trying to figure it out before I got an official answer on that on another thread haha), so the OnlineSubsystem implementation of leaderboards is completely useless.
HOWEVER, following this guide:
You can enable native steam methods and bypass the OnlineSubsystem altogether.
THEN, I basically copy-pasted the “leaderboards step-by-step” code in the steamworks documentation on the steamworks partner website into a new empty class.
Then I made a new actor class that includes that class with blueprint-callable methods that call those methods.
Then I made a child blueprint class and used the methods that way. If you’re doing everything in C++ you can probably do it much more elegantly than I did, but I’m not much of a programmer haha.