When I use swayidle timeout 1 'wlr-randr --output $(wlr-randr | cut -d \ -f 1) --off' resume 'wlr-randr --output $(wlr-randr | cut -d \ -f 1) --on; pkill -n swayidle on a single monitor to emulate xset dpms force off with your addition to output-management

} else {
    Monitor *m;
    wl_list_for_each(m, &mons, link) {
        if (m->wlr_output->name == wlr_output->name) {
            // focus the left monitor (relative to the current focus)
            m->wlr_output->enabled = !m->wlr_output->enabled;
            focusmon(&ar);
            closemon(m);
            m->wlr_output->enabled = !m->wlr_output->enabled;
        }
    }
}

my system freezes completely and I have to reboot.
I don't know if that happens with multiple monitors, if it doesn't the solution is just not to run that code path when there's only 1 monitor.