Hello @sddw, and welcome to Mender Hub.
In the case of stock Raspbian, without Mender, how do you provide updated DTBs? I know Raspbian makes heavy use of config.txt and DTB overlays and I suspect that can be made to work with mender-convert although you may need to do some of the setup manually on the board during first boot.
The new iteration of mender-convert allows you to add custom hooks to modify the created images so it’s possible that may allow you to add your customizations during image creation. And if you are not using the new version of mender-convert, I strongly urge you to do so as the old one is not being actively maintained.
Drew