Thunder is a Flutter app like Liftoff/Lemmur so the tech stack can be somewhat similar, but the UI is much different (personally I think Thunder's UI is very polished but it's a matter of taste), and it's free software like Liftoff.
If you want another open source app, you should also consider Eternity, but in this case not only the UI is different but also the underlying technologies.
After what happened to Liftoff, one should always consider the strength/health of a community of contributors before adopting a software. Thunder has a very active community and a couple of devs maintaining it, about Eternity I don't know honestly, Raccoon (apart from some bugs you may encounter) is not widely used and has only 1 person behind it so it's only for the brave! π