Sidecar to turn iPad into a Wacom style tablet (i.e. Pen Tablet)?
Solution 1:
Your question is based on the false premise that the quote "You can also use [your Apple Pencil] to write, sketch, and mark up documents while seeing the updates live on your Mac." must mean that it should work exactly like your preferred Wacom tablet. That is not the case.
If you prefer Wacom tablets, I would suggest buying such a device instead of using Sidecar - then you can get a product that works exactly like a Wacom tablet (because it is a Wacom tablet).
Regarding your list of disadvantages:
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Yes, there's no generic option for having a "relative mode" input. However, a specific application on your Mac is free to implement such a mode for that application specifically.
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Yes, there's no "hover" feature - and it is not possible to enable such a feature with the current hardware and software. Individiual applications are free to offer "simulated" hover - for example in the manner you describe.
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No, you do not necessarily have to have black bars. By default your desktop "extends" to the SideCar display instead of mirroring - in that case you do not have the black bars. If you want to see the effect of using the pencil on the iPad simultaneously on your macOS display (and not only on the iPad display), you would then want to use an application that allows you to have the same document open in 2 windows at the same time.
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Yes, video data is being sent to the iPad - that's how it works. Specific applications can avoid that by having a "black" window on the iPad so that video data does not have to be transferred, and having a "content" window on the Mac display showing that results.
All in all, SideCar is not meant to be the same as a Wacom tablet. It has different pros and cons. You can avoid some of the disadvantages that you experience by using applications intended for your purpose, as described above. If your favorite applications does not offer such a mode, you would need to ask (or pay) the developer to create such a mode.