The deduplication saves our ass precisely in week 0. The size of updates over the year is negligible compared to the first installation.
So perhaps we don't care so much that the updates to the different schemes are unrelated after week 1, inclusive.
It won't necessarily be easy to set up, but I assure you that the updates over the year are more than a factor of ten smaller compared to the initial download of TeX Live in week 0, which is precisely why it's crucial to deduplicate in week 0 by layering the schemes but not necessarily in the following weeks. It's easy to demonstrate that by downloading a TL 2021 ISO and then updating it.