We have a bunch of these vans and the heat sucks in all of them. One 2018 was blowing luke warm air when it was -30c(-22F) here a few weeks back. I unhooked the heater hoses at the core and tried to flush the core out with a garden hose going back and forth. I also zip tied a piece of cardboard to block the drivers half of the lower grille in the bumper. The heat seemed much better after that.
On older more stubborn vehicles I've had luck with using industrial drain cleaner to flush out copper heater cores. Pour some in the core, let it sit a few minutes then flush it out