This error was later fixed. Skip down to the numbered list in this post to see method to switch from stock A12 and perform upgrades.
Error 21 when sideloading the zip. TWRP also would not proceed past splash screen and other thread sources suggested recovery. Method I just tried:
Recovery mode (Power + Volume down to enter Fastboot mode - volume keys to scroll and power button to select Recovery Mode - Power + Volume up (reovery mode shows after approx 10 seconds hold)) > Apply update from ADB > "adb sideload PixelExperience.zip"
Running A12 already via OTA so attempting this really for the security patches. TWRP 126.96.36.199-0-crosshatch.img. ADB 31.0.3-7562133.
Could I just dirty flash using another (earlier) verion of TWRP or do I actually need to wipe data/factory reset in recovery mode? Or am I missing something else entirely?
EDIT: after all the helpful responses from others here, this is what got this ROM installed on my crosshatch device which was previously running A12 factory image SP1A.210812.015.
1. adb reboot bootloader (I did this via commandline. It is hardware accessible too but not what I used this successful time)
2. fastboot flash boot --slot b lineage-18.1-XXXX.img (this step not required when later updating PE)
3. on the device used up/down hardware keys to switch to recovery mode and select the option using the power key
4. device then shows android laying on its back and required power + up key held down (after several seconds release the up key) to enter recovery mode. This in particular required several attempts before the lineage recovery kicked in. It's jazzy purple so was quick to spot when successful. (this step not required when later updating PE)
5. factory reset device - other users mention not doing this is problematic and was also my experience. You won't be sad for long trust me. (this step not required when later updating PE)
6. update via adb on device recovery mode (when later updating PE it will have switched to its own recovery instead)
7. back in computer commandline "adb sideload PixelExperience-XXXX.zip"
8. (this step is no issue when simply upgrading PE) at around 47% progress the device then complained about signature being unsigned and that the install would be a downgrade. Had to do this to gain ADB sideload access. Switched answer to YES for both. Progress % didn't change but eventualy 2/2 steps completed. I had to do this step several times too. No idea why but across all devices over the years it's a shared experience when installing a new ROM. No gain with pain!
9. final step on the device was to remove the USB cable and choose to restart via the lineage recovery menu (or if upgrading PE it's of course "Reboot system now") - if you updated check the new security patch date in settings >> about phone >> android version >> note the android security update details.
So far everything working but just neeed to understand getting Magisk back for my workflow.
EDIT: Got magisk working using the payload-dumper-go method as outlined here https://www.xda-developers.com/how-to-install-magisk/