That's Birmingham water for you, it's not actually all that bad for keeping amazonian fish in.
To start simply. Ignore the pH for now. It's important, but it won't stay put so I'd quit worrying about it at the stage you are. RO water is almost always just on the acid side of neutral.
If you get the hardness right, mainly the kh, then your pH will buffer nicely. The tank will have a whole pile of organic acids that will bring that right down once you add your tap water, which is how bog woods and peat tend to lower pH. So set the kh to what you want, the gh will generally follow suit and the pH should stay where you want it so long as your maintenance regieme doesn't drop off.
Are you using a hand held tds meter? Or is it inline?
If it's hand held then take a look at the TDS of the tank water, if you want 100-150 then start working towards that, gently, by adding RO water. It'll often keep rising as the gap between water changes progresses.
I use RO in some tanks with remineralisation salts, but that's because my water looks like limestone in comparison to yours.