Here's my 2bits... IMO much of the hype about Mesh is over-rated. I've found the performance to be somewhat disappointing.
In reality, most people will never be "Networking Experts" nor will they typically be "Network Enthusiast" or inclined to learn a ton about all the complexities.
They just want connectivity & decent performance.
MESH is super easy to setup but just as anything with automatic configuration... it doesn't give much feedback or information as to what is really going on.
It sounds like your usage case is... much like my own.
There may be a few dead spots or an end-location completely out of range of one single router.
However in our cases there are likely dead-zones causes by physical structures of our houses & even appliances etc which block & impede radio waves.
Yet if you step to the side... you are still well within range of the first/main WiFi router.
Why I think AiMESH is somewhat flawed is... you can't set the individual signal strength of the nodes.
Logically you would just want the nodes to cover the small dead-zone, & not transmit WiFi all through-out much of the house.
Visualize minimizing WiFi radio-signal over-lap.
And... when you have WiFi radio overlap, any client devices will hang on to the WiFi for as long as possible (Well as long as it's connection algorithm dictates).
I'm not sure if your mobile devices are Apple or Android but for Android...
I've always found WiFi Analyzer does a decent enough job of letting me see the dB signal strength of 2.4G & 5G on my mobile client device.
You described using the default -70 dB
But if you want clients to be forcibly disconnected via the router adjust it to a less Negative value... try -60 (If that's too much try maybe -65)
Anyways my point with MESH is too much WiFi Radio is never great for performance nor reliability.
Probably should turn down the TX power (Under: Wireless>Professional Tab [@Bottom]) to Minimize the Radio Overlap.
But the Big Problem is the Mesh Nodes want to copy the main router's TX power setting & the same goes for the
Roaming assistant value Arrrg!!!
Anyways I'm not sure if your house can somehow allow you to implement it but...
If you can connect the nodes to the main router via Ethernet.
You may find it's actually better to change your node to an access-point.
By doing this you can set the TX power & Roaming assistant values SEPARATELY.
You should be able to pick a suitable Roaming assist value that works with the varying signal strengths on your mobile devices.
Sorry for the novel but, not everything is easy.
Automatic Configs such as AiMesh are rarely optimized or always Best.