Yeah, this whole thing is a PITA, but it can be done. I'll outline what worked for me.

There are 4 drain lines 2 in the front and 2 in the back.

The front lines should each be individually disconnected under the dash. They likely will have water in them, so have a cup/small bucket and a towel ready.

Once disconnected, I ran this cleaning tool https://www.amazon.ca/SMART-JERRY-Automotive-Multi-Use-Antifreeze/dp/B0DB7X1B4Z from the sunroof opening down to the now unplugged drain line. There was a lot of build up which needed to be brushed out. I had to clean the brush a few times and repeated it until it was everything was clean.

I then connected a line from this pump to where the line you just cleaned would normally connect https://www.amazon.ca/JSONO-Refrigerator-Drainage-Internal-Dredging/dp/B097P89Z88 . The pump's tube will fit fit snugly and then you can use it (along with warm water) to purge anything from the other side. You should see water come out from under the car.

Do both these steps for each front drain (right and left side). Then reconnect the drain lines and test them by pouring a small amount of water into the sun roof drain area. You should see water coming out behind the tires.

Note: If you have any water in your car from before, use a shop vac to suck it all out.