I have a Base Unit and 1 satellite unit to cover 3900 sq/ft and about 26 devices connected including laptops, printers, smart phones, iPads, Galaxy Tablets, TVs, Ring devices, Sonos units, smart outlets, L2 EVSE and garage door opener.
23 devices connect to 1 eero while just 3 connect to the other eero.
Eero should utilize automatic load balancing and detection software to prevent connected devices from attaching to just one eero. Otherwise, what’s the point of having additional eero units on the network if just one is providing most of the device connections?
You have an insufficient number of eeros covering that much space in my opinion. Two Pros for example are (only) rated for 3500 sq ft so I'm guessing your leaf node is struggling with a less than terrific backhaul path to your gateway eero. Perhaps that's why most of your clients hang on to the gateway node. Support could check your network node connections and confirm that and offer suggestions to correct the problem.
To answer your other question, client devices don't like being told what node they should connect to. They can be gently pushed towards a particular node but ultimately it's up to the client device.