Forced air Intake Question

500 sq ft condo, Goodman forced air (heat and cooling), only lived here for 4 months. Not super knowledgeable in HVAC, I’ve always owned radiator heat.

I’m in the process of cleaning the vents and trying to improve dust/air quality when I came across cleaning the intake. It’s at the bottom of my staircase and I’ve checked the filter before (just replaced it this time). But now I’m realizing the tight wall space behind it. When I went to the unit, I’m now noticing a spot where air filters normally go on the forced air unit (please correct me if this slot is for something else).

Overall, can I add another filter at the unit or should I only keep one at the unit and remove the intake by the stairs? I’m guessing the issue here is with air flow in such a tight space. There is one more vent to the small closet the unit is in. I’m thinking the staircase intake isn’t getting much use but the vent on the side of the closet and thus the open slot is pulling in most of the air?

Appreciate any advice, thank you.