@thomas166 So there few components to your requirements. The first is that you don't want to have the firmware misimatch issue. The only solution to that is you need to that is to create a DTS for the lte, since sysupgrade reads the device type from from /tmp/bard.json and /etc/board.json, these are created when the device tree is loaded, so you need to create a dts with the correct identifier in it.
Here is a link to a patch I created that will add the lte device and include the lte-base in the build. This is a patch for OpenWrt 23.05.3 so you may have to merge it in manually.