More than five years and I had never measure the l...

Perhaps, it really depends, you could be lucky and have very good water source and not see any improvement. But, if you are spending all that much money in the products you are feeding the plants, I would definitely spend a little on a cheap pH pen and a TDS/EC meter, they really help in (i) feeding the correct amount, (ii) checking you water source, (iii) checking your run off. I only use Biogrow and Bio bloom from the Biobizz line and have got over 1 gpw last grow; well I also use a cal / mag additive because my tap water is too soft and I would never would have noticed that unless I had measured my water with an EC meter. As for CO2, before getting a CO2 enrichment system I would get a better light. Here's a tip for you, the CO2 levels of a bedroom (with people sleeping in there) rises very fast; So if you have your indoor setup in a bedroom and put the 12 hour flowering period during the night, your plant can get air richer in CO2.