CO2 is not going to actually lower your
KH, but your
KH is what will help keep your pH stable so it is not necessarily a bad thing. When you are dosing
CO2 you have to keep a very close eye on pH and
KH, including doing a test in the morning before lights-on to make sure it does not go too high when plants are not photosynthesizing (though with 1wpg you gotta stick with the very low light plants).
To answer your question, I don't think you will get your pH down to 7.2, but maybe 7.4 or 7.6, but I certainly don't know for sure.
DIY will work, 2 Hagens will work, and pressurized will work - totally depends on how much work you want to do and how much money you want to spend. The best way to lower
KH would be to mix with
RO water for water changes.
I like livebearers and Africans so I would like to switch water with you,
LOL!