I think two things are getting mixed up here.
Firstly
fastboot flash dtbo <dtbo>.img
fastboot flash vendor_boot <vendor_boot>.img
is certainly an issue that is less than ideal. I would very much like to see this resolved. The /e/OS build does contain dtbo.img + vendor_boot.img but at install time the components are required “out of sequence”. Users can produce their own dtbo.img + vendor_boot.img from the /e/OS build using this method https://community.e.foundation/t/oneplus-9-pro-lemonadep-documentation-suggestions/36790/8?u=aibd.
Secondly, is berlin, as a 2021 device, still receiving manufacturer firmware revisions ? November 2022, does seem a rather short period of support. [1]
You also see the date of the last manufacturer firmware revision represented as Vendor security patch level here.
Settings > About phone > Android version > Vendor security patch level
There will have been as you say
multiple new updates to these images. the newest one is from 2023-05-20
… but we do not know if there actually is any manufacturer firmware revision included, or available to include.
[1] https://www.androidpolice.com/when-will-my-phone-get-android-13/#motorola
Motorola
Once a Google property, the Lenovo-owned manufacturer has fallen from grace in the software department. It’ll generally stick with two OS updates and two years of security patches for flagship, and possibly one OS bump for other devices.