slack-gradle-plugin VS idiomatic-gradle

You can go very far with this approach. Slack has a single standardised plugin that they apply to all their android projects which exposes some high level config DSLs for those projects to configure which build features they want/need https://github.com/slackhq/slack-gradle-plugin

slackhq/slack-gradle-plugin is also pretty nice as a reference for really streamlined projects

As for the Gradle setup, I've always wanted to remove as much boilerplate in the Gradle scripts as possible, and custom Gradle plugins is one of the solutions I found. There is still some work to be done in terms of cleaning it up, although I am quite pleased with how it turned out anyway. The slack gradle plugin is full of great info about how to create your custom Gradle plugins to centralize Gradle configuration into a easy-to-use API.

You should look into how https://github.com/jjohannes/idiomatic-gradle and similar projects handle it (nowinandroid has a good set up as well).

And finally, look at the idiomatic gradle repo. There is also a free video series that goes with this Understanding Gradle – Video Series. There are lots of bad practices in gradle that will slow your builds down, like using allProjects {}

I'd suggest getting rid of buildSrc, and just using build composition. I mentioned in another response, but check out https://github.com/jjohannes/idiomatic-gradle for how to use precompiled script plugins that are connected to the project via includedBuilds. The only issues I've had with this are generally resolved by just running the sub-build independently to resolve any caching issues in the 'primary' build. A simple gradlew build --settings-file ./build-logic/settings.gradle.kts run will generally resolve that, and that issue seems largely because of the 'root' build being a peer or anscestor of the included build (due to legacy structure of the project, an issue the linked example repo does not have).

When we initially migrated from buildSrc to an included build, it seemed an elegant solution to the problems outlined above. We were dropping buildSrc, with all its problems, and using the modern facility that Gradle considers the preferred, idiomatic replacement for buildSrc. (For a very elaborate example of this, see the idiomatic-gradle repo by former Gradle engineer Jendrik Johannes.) Included builds are indeed the best of both worlds in many ways: they can exist under version control in the same repository as the build that uses them; they can be easily opened in a separate IDE instance, improving build engineer productivity; they don't have the automatic test behavior of buildSrc, which improves feature engineer performance; and they're generally very well-behaved, without any of the magic of buildSrc.