I think you are grossly misunderstanding. OTA updates are not impossible, they are just not implemented. As you are wanting to generate customized AOSP builds, you have no alternative (regardless of the device being selected) to implementing the OTA infrastructure yourself.
This type of device is not, in general, an end-user type product, so the supporting infrastructure for things like OTA’s will not be present for them at all, even for so-called “factory” system images.
If you wish to build it into an end-user type of device, then you would be responsible for implementing the infrastructure required for supporting OTAs yourself.
This is not a hugely burdensome proposition. The AOSP device configuration can easily be set up to generate OTA updates. Start your reading here; https://source.android.com/devices/tech/ota