F-Droid

Worth noting, they support reproducible builds, which allows developers to sign with their own key:

https://f-droid.org/docs/Reproducible_Builds/

I would definitely recommend going this route if you're starting with a new app. Having the binary on GitHub (or wherever you'd otherwise publish) match exactly the binary on F-Droid is really good for assuring people nothing in your repo was tampered with during the build process (i.e. that the binary was built from the public code, and nothing else).

It should not take extra work to do this. The project generated by Android Studio should already be reproducible. As long as you don't change the build setup and break reproducibility yourself, it'll "just work." When you submit to F-Droid, just be sure to let them know you want to go the reproducible route (if you make the PR yourself, it's a flag in the YAML file).