For rendering, Slate is using a library from GitHub called FrameCapturer. MP4 recording was not behaving correctly at that time, and this is the reason I decided to deprecate it. I see though, that the library has recently been updated, which is a good thing of course. I will test the MP4 recording capabilities of the library out again and see if it now works better that it did before. If so I will add mp4 again! 🙂
Tweening options are not available in animate property values (or any other keyframe based parameters), because in animate property values, there is a full curve editor with which you can basically define any type of tween/interpolation you want and is at your total control 🙂
So for example something like this, would be the equivalent of BounceInOut.