Hello everyone,
I just wanted to post an update on this: turns out I am an idiot.
I spent the last 4 days trying to figure out why I wasn’t able to flash this damn thing.
Yesterday, I grabbed a couple of old Shelly 1 and Sonoff S20 that I had laying around and tried to flash those. I wasn’t able. Baffling, since I flashed those exact items in the past with the same USB to Serial converter, same cables, same software running on the same PC.
I troubleshooted a bit with Tasmotizer and discovered I was failing because it couldn’t communicate on COM1 (I was getting an error message saying that I didn’t have permission on that ///COM1 port).
Turn out that my PC lacked the proper PCI Serial Port drivers (I updated it to Windows 10 20H2). I never needed to download anything in the past and I flashed everything correctly. I grabbed the correct PCI Serial port driver from the web, installed those and restarted Tasmotizer. Now I was able to flash the Zemismart Curtain USB dongle on COM3 port and I finally managed to configure the curtains following this helpful post here --> Curtain Automation
I am now able to control it in HA as if it was a dimmable light and it works just fine!