Isn't UE4/5 fully source available?
ID's engines are always called "The world's most expensive xcopy". You always get the source, but you never share it.
Same model. Get the license, get the source.
See the relevant page at: https://dev.epicgames.com/documentation/en-us/unreal-engine/...
From that page:
> This page provides detailed instructions for subscribers to download the source code for Unreal Engine (UE) from the Unreal Engine GitHub repository, and to get started working with the code.
(Emphasis mine)
* The NDA'ed console bits are hidden of course but all the PC code is there. Not just release snapshots either, you can see all of Epics commits in real time.
It’s just free as in beer.
“Click through EULA” is the mammoth in the room.