With a 3-1 calcium ratio some ph rise will be inevitable, my water comes out the tap at roughly ph 7, pretty saure it should take it into the realms of habitable for the rift cichlids., but you can use rock, oystershell and sands to control the ph too. The nitrate level round here is hacking me off too, but since were pretty aquarium ph neutral you can bump things in just about any direction you like quite easily. Ive only used seachem equilibrium which works pretty well buit I assume the hagen one works pretty well too.
TBH withour water you could just go for denitration, full RO might not be necessary.
I have neolamprologus multifasciatus for which London water is fine (very hard, pH 7.8 out of tap so easy to get it 8+ in tank with ocean rock) except tap water nitrate is 20-40 so I swapped my under sink drinking water filter kit for one which includes a nitrate removal cartridge
It's the twin specialist system, 3rd one down. Easy to fit.
I replace the nitrate removal cartridge every 3 months and the other one every 6 months. (But that's doing water changes on 4 tanks). Replacement cartridges are ?14.
Have never worked out how it would compare with buying RO from lfs.
I think theoretically you don't need to use dechlorinator with this either (not sure if that's the case if there is chloramine in water), but I err on the side of caution and don' rely on that as I have not got a test, and I would not want to merrily add filtered water and find out too late that the cartridge needed replacement.