With GH so low I would dose a GH Booster. Depending on the fish, I would add enough to test 3 degrees GH for soft water fish, higher for hard water fish.
Then I would add enough baking soda to make the KH roughly the same. Does not have to be exact.
I do not think cuttlebone 'messes up the water'. Whoever said that is probably thinking that it will add minerals to the water (which it does) and can make the water harder, which some people do not want, but you do want. Sure, add cuttlebone. Some snails will actually crawl across it, and eat a bit, especially if a little algae grows on it.
NLS, like most fish food will add N, P, and most micros to the water, but not so much K or Fe.
If you want to try easing your way into fertilizers you could get bottled fertilizers, such as the Seachem line or any other. But as noted above, you are paying for a bottle of water, with just a little actual fertilizer in it. It is OK to do once, and practice dosing until you figure out what your plants want.
Then buy the dry products and refill the bottles.