Thanks for the detailed reply. I ended up giving up on 11 because I found bugs like the dialer not hanging up a call. That was enough for me to get rid of it as unreliable. I then tried to install 10 again by first flashing the Aug 20 stock firmware and then installed PD10 using the links at the start of this thread.
I had exactly the same issue as the first time I tried to install. The install got halfway through and then gave an error. I can't remember the exact error that was shown in the command window. I should have written it down. The phone definitely wouldn't boot. At that point I gave up for now and flashed stock and rooted it with Magisk no problem.
I'd like to give PD 10 another try because I really like the look of what I saw when I had PD 11 running. Unless I've done something wrong when installing 10 I'm not sure how to get it installed, given that my previous 2 attempts failed in exactly the same way.
Should I be flashing the Aug 2020 stock image and then using the links to the rom zip and boot image at the start of the thread as I have done on my previous 2 attempts?
Edit: It wasn't a slot problem because.....
fastboot --set-active=a or fastboot --set-active=b didn't help it to boot.
The easiest way to know what firmware to use (assuming its not in the OP), is to open the ROM zip, go to META-INF\com\android and read this line"
I highlighted the important lines, then go to the official Google firmware page for the device, and find the matching firmware. This corelates to:
10.0.0 (QQ3A.200805.001, Aug 2020)
I dont think thats your issue though, I think its either kernel related, or you are not patching the boot.img from the same OS you are using, with the same MAGISK installed in the OS. That's the only time I get bootloops, when I do not make a new and fresh patched img . FROM the same installed OS running the MAGISK installed FROM that same OS you plan to ROOT (meaning do not use anything other than the boot.img made from the SAME OS you installed, using the right boot.img from the zip).
You also didnt really answer my questions about install steps and processes used, so I am best guessing based on what you did reply with. If you follow the OP steps to a T, it wont fail, so I am confused what failed, where (halfway through), is that MAGISK, the OS ROM, ETC?
Do this, extract the payload.bin from the PD 10 ROM zip, then use python to extract the boot.img from the SAME PD ROM zip used to install the OS, thats what you patch in MAGISK. THEN take that NEW boot.img (from the original payload) install within MAGISK, select patch, select the boot.img you just extracted using python (payload dumper), then ADB that to the boot. Thats the right process and anything else isnt stable.
This isnt a slot issue so I wouldnt expect that to solve anything. I suspect you arent using python to extract boot.img using the payload dumper scripts (since you didnt note that anywhere but its CRITICAL), then using that dumped boot.img (from payload script) and using the installed PD 10 ROM, install MAGISK and patch that and ONLY that boot.img within MAGISK (once its installed and running in the OS). Its kind of critical you know what you're doing, reach out if you need exact steps and more help.