Synfig Studio – Open-Source 2D Animation Software
synfig.org
synfig.org
Nonetheless... I have no idea of how this really compares to other tools, but when I have had a need, Synfig was a breeze to use for what I needed it for. Importantly, it was quick to get started with, and without needing to spend hours in tutorials trying to figure out how to do things. So happy user and all that :)
[1] https://youtube.com/playlist?list=PLJ1s8hm5LwbTlUt_kOmQ04hK5...
Compared to Lottie, you can make animations for Rive directly in their web editor.
The downside is that you have to create an account to use it.