I see, I run several softwater tanks, they are sweet. I do not see why you would need the acid buffer or humic acid, your RO will be about as clean/antiseptic as you can get without having to put additives into the tank.
Your ph will be around neutral using pure RO/DI, although I would cut it with tap, possibly 9 to1 or 8 to 2, try it with measuring cups see what it comes out as, 3 cups RO to 1 cup Tap, KH will probably be around 6, as hard as your water is, maybe a tad less.
To drop the ph simply add C02 to the tank, don't use anything else, you don't need it, some peat below the substrate is always a plus too.
I run a couple of my tanks with a KH of about 2, ph before lights on is 6.4-ish, once the C02 runs a bit I dont know how low it goes, I don't really care, I use the fish and plants to tell me how things are, learn to use the spraybar or lily pipe for aerational and maintanance purposes on a daily basis.
Running a softwater tank is actually very easy and the plant choices you then have are much better
Your water is liquid rock, you will have to change filters in the RO unit regularly, I would test it about every two to three months.
Don't worry so much about GH, it will usually stay a degree or two higher than the KH.