You already have the custom dtbo.img and vbmeta.img flashed on your device because you already did the /e/OS install in the past.
So from my point if vue,
[HOWTO] Upgrade my /e/OS to major Android version without loosing /data
you can have a go upgrading simply choosing local update from the device → settings → system updater → 3dot menu → local update
This will first replace the current recovery, then install the /e/ build…