First, this vehicle isn't offered in the US, but I will try my best to help.
The most common cause of low heat after thermostat replacement is low coolant or air in the system. Have you tried to purge the system of air?
Next, there is an actuator on most vehicles that controls the air temperature entering the vehicle. It is called a blend air door actuator. If that is going bad, it could be the issue as well.
Let's start with this. Here is a link that explains the most common causes of a heater issue. Take a look through it and see if it helps.
Also, here is a link which explains how to replace a blend air door actuator. I'm adding it so you can gain a better understanding of how it works and in general how one is replaced.
Please keep in mind, I am not familiar with the design of this vehicle, so the blend air door may not exist. However, it is the most common means of controlling air temp used today in newer vehicles.
Again, based on your description, I have a feeling there is air in the system which is creating an air block and preventing good coolant flow through the heater core. One thing most people are not familiar with is something called a jiggle valve. It is a small pin-like device on the thermostat. It allows trapped air in the cooling system to pass through the thermostat and be released. In most cases, it would be positioned in the 12 o'clock position. It's just a thought. I attached a pic below of a thermostat and highlighted the jiggle valve. Do you recall if there was one on the new part?
Let me know if that helps or if you have other questions.
Tuesday, December 22nd, 2020 AT 7:20 PM