For "sideloading", I don't have much of a preferred method, going mainly by what I can find that works.

But aside from the aforementioned Itchio and F-Droid, I also use git-based services like Github, game stores and Patreon-like sites if/when they have Android contents, through Aurora Store if the program is on Google Play only and either I paid for ir already or it's free, and some times I even download the apps directly from the Play Store from another device, generating an APK from it with an appropriate tool and moving to my main phone, which uses a vanilla system.

Also I back up everything I can, and test them in both my main phone while in airplane mode, and in a virtual machine running BlissOS 15 (Android ported to the x86 architecture).