may be more elegant to have something like #106395 but my PR kinda breaks nixos :(

With your approach:

• Merging behaviour can be obtained by setting extraStructuredConfig = lib.mkDefault { SOME_OPTION = yes; }
• Setting extraStructuredConfig without mkDefault will completely disable common-config. This runs a bit counter to my interpretation of "extra" config.

With my approach:

• Merging behaviour remains default
• Individual settings from common-config can be overridden with mkForce, as in extraStructuredConfig = { SOME_OPTION = lib.mkForce yes; }
• All of common-config can be disabled using useCommonConfig = false;

…
and I just noticed that without any changes, one can already override all of common-config using extraStructuredConfig = lib.mkForce {…}…

IMHO your approach is very likely to result in surprises: setting extraStructuredConfig for just one option will suddenly disable a bunch of others.

Closing in favour of #90065 and use of extraStructuredConfig = lib.mkForce {…} to override all of common-config.