The rule of thumb is “anything” provided by Epic as a usable asset falls under the category of “fair use” with out the requirement of additional fees beyond what is provided as to what is required to make a game functional. The fair use fee as to individual assets is generally covered by the fee you would pay as part of the check out process as a pay wall between the end user and the server system that allows for such assets to be downloaded so even if marked as free you have obtained the required license to make use of the asset as long as it’s use is limited as part of a Unreal 4>5 project.
Since you state.
“In my plan, the virtual characters will be rendered in Unreal Engine”
Then the license requirements as to intention is full filled on your end.
Where this would change is if you produced a “product” that allows individuals to interact with the assets in real time as in and limited to a “Video” game then a royalty payment is required on each unit sold as to the use of the Unreal engine for this purpose. Even in this case your use of MetaHuman is still covered as to fair use as to the licensing requirement as to the asset being used in Unreal 4 for the purpose of rendering and is not exclusive to it’s use and purpose of a video game.
Based on the info as to describing your intention your well with in fair use and others have already done so as far as using Unreal 4>5 as a rendering platform and has already been made use of in other industries not relating to video games.