The fan should be chosen for static pressure. Typically static pressure fans feature less angle of attack and more blades.
Sealing is very important. Make sure to use tape anywhere between the filters and make sure that the fan fits well its barrel.
For example, if you're just setting up a simple "fan blows air through filter" setup, more static pressure is needed to force more air through the filter; a fan that can move 30 units of air per minute might not be able to move 30 units of air per minute when an obstruction, like a fine-particulate filter, is placed in front of it.
If you look at the fan used in the article, you can see that the fan blades don't reach all the way to the outside of the cowling, meaning that some air could actually make its way through that gap, and that the blades are large but sparse, meaning there's more room for unwanted airflow; in other words, the air flow you're getting might not all be coming through the filters, but rather flowing around the fan and getting blown back out.
Given that the airflow is only 1/4 of the rated airflow for the fan, it seems that the resistance of the filter is causing a pressure drop across the fan that is fairly close to its static pressure. Adding another fan in parallel to try to move more air likely won't make an iota of difference, as the pressure drop will be about the same. However, giving it a fan with a higher static pressure likely will increase the airflow, even if the rated airflow is lower. Or, add a second fan in series with the existing fan (with appropriate distance between the two to smooth out the turbulence), which could double the static pressure and therefore the airflow. Yes, adding more filters would also help.
There's a reason that most fans in air filters are centrifugal (radial) fans rather than axial fans - they have a higher static pressure.
What you want is a fan that is designed for particular circumstance.
https://images.app.goo.gl/ULuRqRvEA6m3qYnQ8
Since HEPA filter is significant obstruction you want a fan that is designed for static pressure (which is ability to push air against a force/obstruction).
The pressure difference caused by the fan will be different from the same fan but running in free air (not obstructed).