For building specific packages with particular flags, wouldn't slackbuilds have been sufficient?

Reducing the dependency tree gets a bit more complicated once you consider that now you have to satisfy not only runtime dependencies for all packages but also build-time dependencies. There may be ways of cleaning that up after a build, but next time you want to emerge a new package you'll just end up having to re-build the build-time dependencies, so in practice you'll just end up leaving them there. There is an ability to emerge packages to a separate part of the filesystem tree (ROOT="/my/chroot" emerge bla), so that you have one build-time system act as a kind of incubator for a runtime system that gets to be minimal. But you'll end up encountering problems that most other Gentoo users wouldn't encounter, having to do with the separation between build-time dependencies and runtime dependencies not being correctly made in the recipes. Personally, I had been relying on this feature for roughly the last 10 years, but there has been steady deterioration there over the years and I eventually gave up late last year.

That's very fair, I've certainly tinkered with stuff for the fun of it when there were easier or more suitable alternatives.