Alexandra, You would normally install Mac OS Mojave from the App store (update). When I did that, I had a lot of problems with the systems software. So what I did, was a clean install. Installing from a USB stick. This wipes your disk clean and installs the Mac OS. Meaning, I had to re-install all of my software programs again. The advantage: no residual from the old operating systems and software programs. Disadvantage: a lot of work re-installing everything and getting/entering all of the software and keys to make everything work again.
These new tablets (Huion/XP-pen) are pretty new into this field of art/illustration. They are not quite as good as the Wacom cintiq, but there is also a huge price difference. I figure what I would pay for a Wacom tablet of 16 inches ($1500), I could buy my Huion 15.6 inch for less than 1/3 the price. And could upgrade down the road when they start getting 4k graphics and pens. For now, mine works great for what I paid for it. I love having the ability to draw on the screen that I am viewing instead of drawing on the tablet and looking up into another screen. Makes drawing a lot easier.
The past is history, the future is a mystery, all you have is the present