Hope you haven’t given up in despair by the time I write this @Franko. Here are a few ideas of mine.
Having installed /e/ on two different 5T devices, my advice would be to update to the new version 10 of the stock ROM - download from here.
You can then download a version of TWRP by mauronofrio who seems to have the best reputation for for adapting the TWRP recovery to Oneplus devices. I would start with the older 3.3.1 version which worked best for me, but perhaps after updating to version 10 of Oxygen OS the later version may work.
This page on xda-developers will have more information on which version to use:
I had no difficulty in using ADB with fastboot to flash twrp onto my 5T.
You have to reboot into stock rom temporarily to re-enable USB debugging, then reset the permission when you plug in the cable to your computer to allow ADB access. Remember to reflash TWRP after this to replace Oxygen’s recovery.
If you fail this way, try this to boot into straight TWRP:
fastboot boot twrp-xxx.img
Assuming you have the image version. If not download a copy of latest TWRP image from here.
Or else start your phone into bootloader mode by holding volume down button at the same time as power then manually boot it into recovery by pressing the Vol Button for a couple of times till you see recovery in red at the top and then hit the power button to reboot.
If you have an OTG connection to a USB stick you can inverse the operation by booting with the image file first and then using the temporary install to select the USB drive and flash the zip version by mauronofrio from there. But the same holds true, when you leave the temporary recovery select reboot into recovery to get back into your permanent version of TWRP.
AT this point it is essential to to a clean wipe by formatting data to get rid of the encryption of OOS, then also Advanced wipe to clean Dalvik/ART cache, Cache and System.
Two choices after this:
- Use ADB to sideload the /e/ zip file
- If you have copied it onto your USB stick then flash directly from there (probably the easiest solution)
Of course to allow sideloading, you have to go back to the main menu, choose “advanced” then “sideload”.
One of the best guides I saw was done by a Oneplus 5 user @manish here on the community site.
Although for the earlier model, by just changing the references from “cheeseburger” to “dumpling” the whole procedure is the same. Hope this helps.