@ExNull-Tyelor @FCSW_Ben FYI, the spine-unity runtime on the 4.3-beta branch has been updated to load 4.3 skeletons from the most recent Spine Editor now. The previous spine-unity 4.3-beta state reading skeletons of version 4.2 was just mirroring the state of the 4.2 branch, without any 4.3-beta changes included yet.

The first 4.3-beta spine-unity packages are available for download here as usual:
https://esotericsoftware.com/spine-unity-download
Please note that as always, any beta (especially when early in development) is not recommended for production use.

We are currently investigating an issue with negative skeleton scale and Transform Constraint changing behaviour compared to 4.2, so you might want to wait a bit before upgrading your existing projects.

Hey @FCSW_Ben
Also an aging animator here, with 20+ years of experience in classical hand-drawn 2d animation and 13+ years of 3d animation.
We try to make Spine as user-friendly as it gets, trying to fit the needs of different animators with different experience levels coming from different software. Spine is unique because it has 2 graph views the Graph and the Curves view. We noticed that some of the old users prefer to use the old Curves view, and there are certainly some use cases where it's more handy to use it than the Graph view.
We take every feedback very seriously and we always try to make our product the best there is, so concerning that we are trying to make the curve behave in a more efficient way.
That being said, I did notice some mistakes in your approach that I think you need to address. The timing chart and the curve representation you drew out do not match. I sketched out the corrected curves on your drawing

To be more clear I did the same thing in Spine with the whole curve visible in the Graph view for clarification.
So from keys 0-6
then 6-9
and 9-13
Another benefit of using the Graph view is offsetting the X and Y curve, which can help a lot in different places. I recommend checking this video, maybe could help:
Also as you like to think in poses rather than frames, as I do, I recommend checking this video where I go through my workflow and show how I think and do animation:
I can see a few different workflows you could try if working with default bezier does not work out for you, but personally, after trying all kinds of different approaches, I still find the fastest and the most efficient workflow the one I explained in the video.
Another thing you are doing that I would avoid is separating tangents for a breakdown, if you want a breakdown to have a smooth transition, you would not separate them. Or separate the tangent on a first key, which has only half of a tangent handle to begin with.
On your example, you show interpolation between key 0 and 5 and then you move the tangent handle to 90 degrees, it's to be expected you will get an extreme overshoot in that case. I recorded the gif making the same thing in Maya so you can see the same curve overshoot result.
I would definitely recommend you to get familiar with a Graph view because you will see what the curve is really doing, and it will help you to notice mistakes right away.

@FCSW_Ben The bugfix has been ported to spine-csharp as well, a new spine-unity 4.2 unitypackage is available for download:
https://esotericsoftware.com/spine-unity-download

@FCSW_Ben Your log cuts off the first line, so I can't see the launcher version. There seems to be 2 problems in your log. One you can solve with instructions here. The other, I'm not sure, it may be caused by the first. Please let us know if you try that and are still having problems!