FYI, it's been a while since you can get a signing developer certificate for free and sign and deploy any app you want for your own device. You can't easily distribute it though, as the complexity is equivalent to sideloading. It's a fair point that Android supports non-Google Play app stores and "Unknown Sources" app downloads. Google Play Services is a a very key tie-in though that's hard to avoid even if you're on LineageOS.