There is a solution:

Main info

Another way of applying system-wide changes in build-time other than using dconf is to use:

Gschema override

What is the advantage of gschema override?

it's a cleaner implementation which doesn't alter dconf database
it doesn't rely on post-setup systemd service for updating its settings
it supports useful logs for troubleshooting (warning if schema is depreciated, warning if schema key is no longer valid)
it's a solution which most major distributions use for deploying modified system-wide settings
you only need to make 1 file for changes compared to making dconf profile & dconf keyfile

What it lacks compared to dconf?

locking settings support
relocatable schemas support (used for making & populating folders f.e.)

in dconf, relocatable schema appears like a classic schema: /org/gnome/desktop/app-folders/folders/Utilities in gsettings, it appears like this: org.gnome.desktop.app-folders.folder:/org/gnome/desktop/app-folders/folders/Utilities/

Rare amount of options are like this, so most maintainers will avoid it successfully.

How to use those?

Gschema overrides have very similar format to dconfs with some little differences. Gschema uses gsettings backend which uses . format instead of dconf's / for schema locations.

While dconf relies on dconf-updater service, gschema override relies on simple command for recompiling gschemas, which is done in build-time as a script after gschema override is placed (this is where useful logs are printed): glib-compile-schemas /usr/share/glib-2.0/schemas

Gschema override is placed in this location: /usr/share/glib-2.0/schemas

It can be named like this: 01-mysettings.gschema.override

It can be also named without a number: mysettings.gschema.override

It is important to pick the name of gschema override, which will assure that it won't get bypassed by other overrides. Alphabetical naming has advantage compared to numerical you seen in example above:

00-99 (99 takes advantage)
a-z (z takes advantage)

So, if you name your gschema override like this, you will be assured it will always work: z1-mysettings.gschema.override

zz- prefix will take the most advantage, but it's preferable to use z1- prefix, to make some space for downstream maintainers to use their own modifications.

Universal Blue images will use z0- prefix in the future, so that's the reason why z1- is preferred for downstream images.