[Essential Phone ROMs, Kernels, Recoveries, & Other] Syndrome666:
OP instructions work perfectly. I got the factory image set up and flashed manually per the flashing instructions in the flash-all.sh file. You need a ”back to stock” image accordingly matching the ROM version of Android (i.e. Android 8.1 correlates to RR 6.0.0), the build number doesn't seem to matter so much, at least I haven't found any bugs as of yet.

Build 4 (latest RR build) is based on version .029 of Android 8.1, however OP provides a .092 essential firmware build. I made my own back to stock zip out of the full dump of .166 of official essential firmware. It really doesn't seem to affect the security patch level. Even though I flashed the lastest .166 firmware based on May security patch, the ROM still says April 5 security patch level. So it really doesn't matter too much, as long as you use one of the lastest back to stock zips, flashing RR over that will work.

Anyway grab a back to stock image, you can use the one provided by Ashwin in the original post, or you can make one yourself out of the back to stock zips available in the stock dumps thread under essential PH-1 xda forums, or just download one readily available there (ones without boot, vendor, and system). If you want the lastest no vendor, boot, system dumps, you'll have to make it yourself from the files listed in the flash-all files as they're not always readily available.

Do not follow the flash-all instructions if you sourced your image from essential's official page, it doesn't include a few necessary files, and the flash-all instruction set in that zip is for a single slot flash, don't recommend using that one unless you know what you're doing with it.

Once you flash the official firmware dump (without boot, vendor, and system).

Follow op's instructions to the T, everything in there worked for me after I manually flashed the official firmware (no boot, vendor, system) files found in the zip.

Again, the instructions are in the flash-all.sh (for Mac or Linux) or flash-all.bat (for Windows). If you open those files in a note pad, you should be able to view the sequence of files to flash. You must flash to both a and b slots for each file, don't skip this and just assume it'll take if you just flash them without the a and b slot designations. That's how you flash to go back to stock firmware only.

The only thing I'd suggest to look out for is when it says to reboot to recovery after flashing ROM, if you simply reboot to recovery, you'll wind up in slot A which had the lineage recovery, and that's not what you need.

Either reboot to bootloader

./fastboot getvar current-slot //output should be slot a, we need it to be on slot b because that's where TWRP is installed.

If your in slot b, just reboot to recovery from device bootloader, if you're still on slot a, on your computer terminal or command prompt window set slot b to active:

./fastboot set_active other

Once you confirm your in slot b, on your phone's bootloader menu select recovery and boot into it, this should boot you back into TWRP.

In TWRP head to reboot menu, switch back to slot a as that's the slot you want the gapps and ROM and root on. Slot b in this case is used solely for TWRP recovery.

Once you set slot a as active, go back and flash the nano gapps (arm64, for Android 8.1 on opengapps).

Once done, wipe cache/dalvik and reboot. All the failed to mount and other error messages are normal, it only says that because data, system, and vendor images are missing from the original firmware today you flashed before applying the RR custom ROM, so don't feel alarmed when you see a bunch of red error messages during the custom ROM flash.

Finally reboot, set your stuff up however you like (restore or as new).

To get root access, follow OP's rooting instructions. Don't follow mine or [MENTION=3462598]BakedTator[/MENTION]'s instructions, they work, but flashing that way (which doesn't include official firmware flashing) breaks deep sleep mode when you're not using the device. Flashing as per OP's instructions breaks nothing of that sort (at least I haven't noticed the same problem yet). My device has had great battery life and deep sleep works like a charm.

I'll update if anything changes, but so far so good.

I reset the frequency timers on CPU spy app when I took the phone off the charger at 100%, currently at 74% with 2 hours 9 mins sot. Deep sleep definitely working like it should.