I find RPM specs to be pretty frustrating. Easy cases like packaging GNU Hello is fine, but things get out of control really fast once you fall off the happy path. RPM specs are based simple text substitution. It lacks the flexibility to make adjustments to the build, the power to build good abstractions, and comes with all the same drawbacks of the C preprocessor. On top of that, documentation is lacking for basic macros, language ecosystem support is spotty and scattered across various obscure projects, and SCL packaging is a complete nightmare. The whole design seems dated compared to other solutions.

Write a build script in your language of choice and then call it from RPM spec.

At that point, why even use package managers? That approach won't benefit from existing packaging infrastructure at all. Might as well distribute binaries in tarballs instead for simplicity.

Slackware uses tarballs, so use Slackware instead. :-/