Just my two cents, but I think autoinstall(8), install.site(5), and
rc.firsttime(8) would work a lot better for mass auto installs of
OpenBSD for customized (or 'desktop') installs.

If a person isn't doing mass installs then I don't see why it would
be needed to install a tool to install a desktop to save time when
its easy as:
$ doas pkg_add mate-desktop chromium
$ which mate-session >>.xsession