This sodium bicarbonate is why your water Kh is so high.
Huh? Have you ever had a water softener?
A water softener does not raise KH. At least not any one that I have ever had, and that's a few.
And you should start a journal. I looked through your posts and couldn't find a single picture of your tank. I'm sure many would like to like to see your tank and follow along.
So, my RO water will have zero KH. Should I mix in tap water to get the KH up to 3 or 4(I’m not even sure if 3-4 is the proper level)? Or just add Baking Soda to accomplish this? I’m asking because you say the KH is high due to NaHCO3 (which is what Baking Soda is). Or might the tap water possibly have some other unwanted things as well?
If you are going to the trouble to use RO water, I would dose to the levels you want to achieve. You can get to any KH or GH you desire, and you can also control the ratio of Ca/Mg. And who knows what else is in the tap? So when you start clean with RO, you can control everything.
Personally I wouldn't use Baking Soda, as why add sodium? Sending it through the RO system just removed it.
Better to use K2CO2 or KHCO3 for KH, and MgSO4 and CaSO4 for GH.
KH anywhere from 2 to 5 is fine, and GH somewhere about 6 to 8 seems work well for most.