Grab the heater hoses at the firewall, if they are hot when engine is up to temp its not the thermostat, its the plenum door actuators in the dash.

I just drive around with a jug of diluted coolant and top up when the low coolant messages comes in. Does not take longer than 20-30 minutes.

vtl if you have any wisdom to share I’d appreciate it. My wisdom, after I dumped more than a grand on finding a cheaper alternative to "expensive" Volvo OE fluid says: use Volvo OE. It costs about $60 for 4 liters in US, and healthy transmission always works best with it. How cheaper than...