I don’t think so, but I can’t get RSSI value from z2m, I think it’s only available in ZHA.
Look for “linkquality” for the device. It is disabled by default.
No, no difference at all - the software you use has no impact on the mesh, and the behaviour of devices.
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That’s the LQI though, not the RSSI
So this is the house :
In red, the coordinator, in orange routers.
And now the LQI dashboard :
All devices with plug icon are routers.
I think everything is here.
Devices offline actually :
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Wow! That’s really impressive. 
My first thought is that the sensors in the two fridges and freezer may be a separate problem. Something to do with being inside a metal box, perhaps?
If I were in your place I would spend a couple of weeks now plotting the positions of the devices which you have to re-pair, to see if there are clusters. If there are, you could try swapping the routers in that area with routers from elsewhere. If the cluster moves too, there’s your problem. If not, experiment by moving in an extra router from elsewhere to see if that helps. And as @Tinkerer says, not all routers are equal.
An unrelated thought - do you use zigbee groups? And if so, do you use automations to send commands to them? Apparently this can overwhelm a zigbee network very quickly - messages to a group get blasted out to every node in the mesh just to make sure all the members of the group get them. Again, I think this is a zigbee thing that would apply to Z2M as well as ZHA.
it took me longer than I thought to build it 
I agree on this one, but they are really usefull when door is left open and temperature raise too fast, I can receive an sms !
I will try to move an ikea plug (they are known to be good routers) closer to some devices that are more often subject to lost network.
I don’t use zigbee groups, I wasn’t aware of this feature.