My 2015 has less heat on the passenger side (measuring air vent temps with a thermometer) but it has leveled off over the last couple of winters. The last "extreme" cold test I ran a few years ago (when it was -4F) the passenger side output barely reached 105F, while the driver side crept up slowly to 130F. The heat output on my car is probably not adequate on the passenger side for extreme cold.
For southern Ohio, my heat is good enough. If I had to keep my wife happy as a passenger in a colder winter climate, I would probably be experimenting with pumping various solutions through the heater core to free up whatever is in there. I can tell you that I'll try anything/everything before I consider replacing the heater core.
The OP having issues with a 2018 is a bit of a head scratcher. Hopefully he can cut his old core open and tell us what it looks like inside.
It's kind of a bummer to hear that the Thermotec heater core may not be a slam dunk solution.
View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)