Maybe that's already there (it was at least discussed on matrix IIRC but I did not find a bug):

With the phone laboratory and green are overlays over landing/green. So every update to landing can break laboratory and every update to green can break purple.
This is problematic since we want to ship e.g. a pathed gnome-control-center which would be broken by newer packages introduced in Debian -> landing -> green.

To fix that it would be good to keep date based snapshots of green around so that purple could overlay a date based snapshot instead of a current version. We could then switch to newer a newer date based snapshot after testing. This has worked nicely in other projects before (that maintained a far bigger delta than what we have with the phone atm).

This has security implications (https://tracker.pureos.net/T602) so we should make sure we only have to support at most one snapshot. This might also help other users who want a less frequently changing base.