Internally we’re already using source built stuff - as you said, unneeded stuff would just have to be removed and our resources added to the editors (meshes, textures, BPs, …).
The problem it all boils down to is that we aren’t allowed to hand that “SDK” out to just anyone. We’d like (and are required) to make sure that only people who’ve agreed to the EULA have access to it so we meet EPIC’s terms.
The only two ways of distributing our SDK are the Launcher (people download the editor and open a modding project) or - what’d be more complicated and thus not really favorable - modding the source after forking it on GitHub to make sure only licensees have access to it.
Method a) has the downside that we can’t use any engine modifications since people would have to use the official builds.
Method b) would pretty likely be way too complicated for most people just wanting to mod the game.
Long story short, that’s why I am trying to evaluate every possible and legal method right now so we can find the one that’s the most comfortable for our modders.