Good progress to be left with just a "simple" overheating problem now . Based on what you wrote, it definitely sounds like some type of blockage which is causing a flow reduction. Water is moving, but not fast enough to keep the temp where it's supposed to be. The heater core must be functioning as a heat sink, pulling off the excess heat and keeping the system in balance.
So here are the possibilities that come to mind. Broken water pump impeller - unlikely, and just about impossible if it's a metal impeller. Thermostat not fully opening - not as likely being fairly new, but possible. Radiator becomming defective - not too likely but possible.
The one other thing that I can think of is the possibility of a small object dropped into the coolant tank while you were topping off. Those aluminum sealers on the coolant jugs need to be peeled-off or punctured before using. I'm sure you're careful, but is there any chance that a piece of that aluminum (or anything else for that matter) dropped down into the jug? Some small object like that could have perhaps flowed to a location where it's now causing a partial circulation restriction. It seems like a smoking gun that the overheat started only after you began to top off the low coolant.