So TH3D has come out with a version of the universal firmware for the Plus. I’ve used their firmware on my printers for years and it’s always been better than factory on the smaller brands. Unfortunately, the lovely screen isn’t supported so you have to buy a screen kit and go back to the days of the old monochrome Ender 3 style with the click knob.
There is working 2.1.X firmware in existence. However, it is not available to anyone yet and is in the process of being cleaned up so it can be added to the main Marlin fork. While TH3D is an option it’s one that cost money. The stock firmware works and I’d advise staying there if you want to keep the current screen.
The TH3D Firmware you are talking about, only works with TH3D replacement board. You cant load TH3D firmware on the stock board. He states that in his online video when He shows how to replace the stock board.