in 2.5 years u haven't taken notice to growth rates........no one has stated anything on light strength only LED....lot more to it then 4-5 weeks, imo.
your going to be exhausting this cabinet? if so why bother with it being air tight...negative pressure brother....as far as light i wouldn't worry about small leaks at all......unless your worried about light escaping from your cabinet.
might be better off building 1 big room with smaller flower n veg inside that room with plenty left for a lung room. it will be cheaper heating an insulated lung room then whole un-insulated garage.