Few aspects forgotten in this discussion:
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Each “annual” adaptation is based on the current Android. 10II is using Android 10, 10III is based on Android 11. This shifts goalposts every year and porters have to work around it.
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BLOBs are provided separately for AOSP and Sony’s own Android by QCOM, if I am not mistaken. As far as I understand, there is a partitioning of the development teams in Sony with proprietary Android and AOSP support developed separately. There is probably some communication between teams, but it is probably restricted to avoid any possible copyright issues. AOSP team is smaller and starts work on the devices when they are released already with proprietary Android.
Take part 2 as my understanding of the affairs. It maybe inaccurate or even wrong, then I hope someone knowledgeable will correct it.
But, as you can see, 1 & 2 combined lead to significant delay. Unfortunately, as discussed many times in this forum and at TJC, Sony seems to be the only manufacturer providing such legal pathway for basing alternative AOSP based ports on their devices. So far, if memory serves me right, nobody was able to list manufacturers that would allow you to downloads BLOBs from the manufacturer site and providing legal framework for flashing devices. If there are alternatives, would be good to list them. Just before making a list, I would suggest to read Developer's Intro · Open Devices to see what such BLOB distribution should match.