I just had to downgrade from EMUI 11 to EMUI 10 to begin the googlefier process. It's actually very simple to downgrade to EMUI 10 and then to 10.0.1.0121 (I'm typing from memory but its the firmware available for download from this thread.
The simple way to downgrade to EMUI is through HiSuite that you download from Huawei's website. No need for HiSuite proxy and other shenanigans.
Use the official HiSuite. Once you connect your phone and follow the whole PC and then Phone process your phone will be recognized by HiSuite. Hit the update button. HiSuite will tell you that y have the last version but will give you the option of downgrading to 10.0.1.0176 (again I'm typing it from memory). Once you've done it (it will take about 30 minutes to downgrade) you should find yourself with a "brand new" P40 Pro with EMUI 10. From this on you can downgrade using the OTG/USB method with the 10.0.1.0121 firmware in the dload directory.
Word of advice once you start the googlefier process as I too struggled quite a bit before nailing it (and now I googlefied the same phone 2 times, the second time on the first shot) :
- The only file you need is linked in this thread. That's the only apk you'll ever need (besides the correct firmware)
- Follow exactly what the hints/popup will show you, if need be note them down.
- When the popup tells you to open playstore a couple of time just do that and nothing else. Don't forget to close/kill the app when closing it.
- Googlefier is not quite clear IMHO about that but one of the very first thing you must do after changing the date is to run LZsomething app (the purple G that will appear once you've restored the backup in step "0")
- Once you've run the LZ app make sure to allow and active. You don't need to run anything else on the app, it sits happily in the background and doesn't need to be run again after the first reboot asked by Googlefier.
- When Googlefier tells you to install something, do just that and hit "done" when done. Don't open anything.
- Lastly, do NOT pay attention to those pesky google warnings about unsecure and uncertified play device. This will disappear one or two step further down the road (provided you followed EXACTLY what googlefier asked you to do).
Be patient, surround yourself with calmness and DON'T rush it. You can do it