Hello! Splitting into multiple documents is indeed a good idea when the config starts to grow, especially when you can find a partitioning such that there is a clear division of responsibility and/or update cadence.
In the SDK this is exposed through declaring multiple config build sources as members of your custom game config build parameters, and then specifying the source per config library. This is documented in
https://docs.metaplay.io/feature-cookbooks/game-configs/using-multiple-sources.
Let us know if document isn't clear enough or if you need additional help otherwise!
Cheers,
Antti