You make a good point there. In normal use, the dropper is expected to hold up 50kg+, so it should have no trouble supporting a ~22kg bike.
I guess the only risk is damaging the post surface.
I don't think it's a knock-off, just one of many manufacturers using Shimano gear to build up a bike. Rockmachine appears to be one of the brands that Bike Fun produce.
I have the same bike. I'm pretty sure you can pull out the 2019 RideControl and put in a 2020+ one which has Ant+. Then you can buy one of these bad boys:
https://www.pushbikes.co.nz/products/giant-ridecontrol-ant-lev-computer
Also there is a thread over here about upgrading a 2019 Trance to...
My gut feel, based on pics from the Giant website, is you will be going around the back and under the motor. To be sure, you will have to probably remove the covers and the motor to see what options you have for that frame.
Deores would be more than enough. I also went from Tektros and the difference it night and day.
You can spend more on Shimano or SRAM or Magura or Hopes or whatever, but it will be diminishing returns, IMO.