Here’s an update:
Stopped treating the tank so I had to replace the Seachem Purigen packs back in the filter.
I may have just stumbled upon the cause of your woes. My 406 was suffering from similar problems. Whilst cleaning earlier to day, I noticed that the plastic impeller cover was warped into a U shape. I immersed the cover in some freshly boiled water to soften the plastic, then bent it back into roughly the correct shape. (Slightly over doing it so as it would sit tight when back in position.) and then run it under a cold tap to harden the plastic again. Instant improvement. I hope that this is helpfully to you.
Per the advice - I checked the impeller cover - It didn't look warped. I cleaned it pretty well and used a little petroleum jelly on the rubber so it would seat better in the case.
I also removed the intake first long sponge because I have a theory there is quite a bit of bypass in this design and I'm going to test my theory:
The long sponges were clean (I squeezed them out in a bucket of tank water) no leaves, no solid pieces yet the "bio" sponge was covered with small snails and quite dirty. Furthermore, I found leaves stuck in the Seachem Matrix in the middle trays and on the "bio" sponge but none in the first sponges - at least some leaves should have been caught in the first sponge if they are doing their job, so here goes...
The flow seems okay - I want to see how well the filter keeps the tank clean and how it looks on the inside after a month. I'll test flow before and after a cleaning.