I quite agree with @OnePlus6user that the /e/ installation on this phone with its A/B configuration is of another order of difficulty compared to earler phones. Having installed /e/ on several different models previously I realised after reading the install guides here and elsewhere it was not going to be a cake-walk with this device. Having read the long exchanges that @Lance provoked with his earlier solution I can see that my experience might put others off!
What is confusing appears to be the relatively simple approach in some guides where they just do a clean wipe after unlocking the bootloader and installing TWRP and then proceed with a straight sideload of the custom ROM onto the first available slot. While this may have worked for some when the 1+6 first came out, it seems that the updates to the OxgenOS to version 10 have created bootloop problems.
Not being a specialist I can only observe that problems seem to arise because of a more complicated boot_control system whereby OTA updates must know which slot to boot from and if they fail a safety check then they can go back to the older already-installed system. Better experts than me can explain this more clearly no doubt. But my guess as to what causes bootloops is that an incorrect overwrite of the boot image on one side throws a spanner in the works and so the added steps of flashing to one slot and then the other in the right order, somehow allow the boot control to function correctly.
It’s for that reason that I decided to follow a longer procedure found here, that flashes the latest OOS update twice successively with reflashing of TWRP in between to allow a return to their recovery and prevent a return to stock recovery. Only after this is the e.zip sideloaded and installed.
Unfortunately, I missed a final reflash of TWRP and ended up with the e-recovery, and with its limited functions it seems to only be able to do updates by adb sideload and cannot flash zip files loaded on internal memory.
My attempts to reinstall the TWRP recovery failed as it seems A/B devices do not have a recovery partition and when I try to flash a recovery it returns “no such partition”. But I was also blocked by the fact that the usual “fastboot boot” method with the twrp image didn’t work anymore. The “boot.img” gets sent and response is OKAY, but the next line says:
Booting Failed (remote: 'unknown command')
I was reluctant to sideload a zip file of the same twrp version which worked before as I have read that it loads it into the system in the same way as sideloading an e-zip file. Perhaps I’m wrong here.
I tried a suggestion of @chrisrg from the Lineage install page for enchilada which said to:
fastboot flash boot twrp.img
Then I had a problem with which slot so I did the same specifying flash boot to each slot, but result was a failure to boot. But at least I had my twrp recovery back.
At this point I decided to restart the whole installation procedure with installation of OOS to both slots followed reinstall of twrp each time, then finally installation of the e-zip file but this time reflashing twrp before restarting the system.
As you can imagine I spent the best part of a day with all these repeated procedures, however, I have at last got a working /e/ installation with Android 10. Oof! 
I would like to be able to root my 1+6 with Magisk to experiment with backup procedures and see if I can get it working like my 5T, but I fear too much modding will make it difficult to get any OTA update working. My apologies for this long harangue.