cs-tween VS spine-runtimes

Get them from Robert Penner's easing site instead. Or, even better, use the normalized ones from flux.lua since they're easier to work with and understand. You can pass them to any lerp function to blend vectors and colours too. You can use this generator to create versions for your language of choice or use these C# versions.

You can make something slide onscreen and scale up; and then half a second later scale down. Coroutines make it easy to write code to do those things in sequence. Find UI animation that's written as code instead of in the Unity Inspector easier to understand and debug.

A "custom tween engine" isn't that big an ordeal, but making something with all the features of dotween certainly would be.

cs-tween for C#

There are a bunch of programs that use rigged and meshed 2D images that create these sorts of animations. I have only used “Spline” for game animation http://esotericsoftware.com/

We're using Spine2D http://esotericsoftware.com/ It's really great

It’s rigged 2d animation. Look into spine. http://esotericsoftware.com

That's easy. Spine Pro.

You may have to learn spine separately from their tutorials and then figure out the godot integration from their sample projects: https://github.com/EsotericSoftware/spine-runtimes/tree/4.1/spine-godot/example

From the first glance it seems like Spine provides their libraries in source format (under non open source license) with pure C++, C# and TS implementations that don't depend on binary blobs (at least it seemed like it). You can find it here https://github.com/esotericsoftware/spine-runtimes . Even if C++ version is not safe, C# and TS versions are probably fine and, and the license mentions creating derivative works (assuming you follow the Editor license terms and some of invovled parties have an editor license) so you are probably allowed to create modified library which processes the files safely.

For example Spine: http://esotericsoftware.com/