Please tell me if it is possible to use a dedicated server for real-time network game together with a third-party mactmaking server?
The idea is to make a backend server on Golang (matchmaking, leaderboards, lootboxes, etc.), and use the UE Server only for the game session.
Yes.
This is a common approach to handling Multiplayer games in general and Unreal Engine is no exception.
Some third party solutions include Amazon AWS Gamelift and Microsoft Playfab
Epic Games is also working on their own Matchmaking Solution called EOS