improve sbt build setup?

There are multiple build.sbt files instead of having 1 that defines the submodules.
The key advantage of having 1 build.sbt is that you can better control the dependency versions. The current build.sbt versions often use the same dependencies but with different versions.
It's also easier to maintain sbt - eg having just 1 build.properties file.
https://github.com/apache/pekko-http is an example of an ASF project that uses sbt and that manages multiple modules within the one build.sbt.