I think if your project is free / open source (edit: and you don’t use LLMs/AI), Codeberg is the a good starting point at least. You can move on to a self-hosted instance if you feel you’ve outgrown it, but even for larger projects I think you can get away with self-hosting just the CI runners.
And don’t forget to donate! https://donate.codeberg.org/ / https://join.codeberg.org/