Yes, you are right, reducing lighting reduces the need for fertilizers.
However, I would be a bit apprehensive about doing this. Plants thrive in a stable environment (especially when it comes to carbon). Suddenly decreasing the amount of carbon and light could still result in problems. Plants adapt to their circumstances to make the best use of what they have available, change is almost always a bad thing for plants and takes time to adapt to it. I don't think you would gain much by keeping your lighting high, if it means you have to start messing with it later on. If you are going to do this, I would test it a couple of times before you actually go away. Any change in conditions can cause problems so beware.
If you did decide to do this, I would definitely not reduce your carbon even if you lower lighting and other ferts. Reason being that plants adapt to the amount of carbon available, and if you go from high carbon to lower carbon, plants basically stop growing completely as they adapt, which can take a week to a month to do. During that time algae can easily start taking over, as the plants are suddenly at a standstill in growth with their handicap.
Another issue you have is that not all plants will do well under lower lighting. So plants that appear healthy, if you were to suddenly reduce lighting, could die off. Plant die off = algae, algae = more plant die off, and it just snowballs.
It's a very risky idea. I'm not saying it can't work, but in practice I would be surprised if you didn't have some problems.
A better idea would be to get your gf into planted tanks....
