You ask a good question. My first thought is don’t mess with something that is working! But keep the source routing and some of the other possible option in quiver as your network expands. In @mrakar 's case for reasons TBD source routing seems to be a must have. Is it the devices and their quirks? Is it how the network was ‘built out’? Much to learn! Good hunting!
Yes, thanks. Lots to learn!
This is a good 
&|:wine_glass: read (pdf):
https://www.silabs.com/documents/public/user-guides/ug103-02-fundamentals-zigbee.pdf
This paragraph:
By nature Zigbee devices are RAM-constrained, but often Zigbee networks are dense. This means that each router is within radio rangeof a large number of other routers. In such cases, the number of neighbors can exceed the maximum number of entries in a device’sneighbor table. In such cases, the wrong choice of which neighbors to keep can lead to routing inefficiencies or worse — a disconnec-ted network.
Might explain my situation from yesterday, all devices (36 of them) were visible to each other (with various signal levels, but still visible) and this might have created my problem. If my devices were further apart so that devices on the edge can not see each other, than my network might have worked. Instead, it might be that source routing (as suggested) is actually a better fit for my particular space in which my devices operate. Interesting (and so very different from wifi).
Interesting yes. However, I am struggling a bit to buy in to the idea that you are the first person in the Zigbee universe to have 36 bulbs in one area. If so, I hope they give you a tee shirt or something. One of the draws is that zigbee has been around for a long time (in tech years anyway), in reading the change log for the chip set in the in Sonoff Zigbee bridge, sure seems to be quite a bit of churn occurring.
I don’t see how you can guarantee how devices ‘see’ each other. Sure seems like they should be able to figure this out without you running around wrapping bulbs in aluminum foil 
Wifi, and specifically WiFi mesh, a whole other animal I think. But with ‘fangs’ as well.
Hoping for a ‘stable mesh’ not a ‘stable mess’ for you!
Agree (fully).
But, according to other posts, some concentrators such as CC2531 have hard limit on how many direct connections they can handle and this number is approx. 30. Maybe, there is similar limit for EZSP as well? It would be interesting to know.
When I would look at the network diagram, I would, for example, see E27 bulb which was disconnected (status: unknown), but last seen just minutes or seconds ago.
I will check later today to see if the lights after 24 hours of communicating among themselves still work as desired. I also have to see if there is some advantages if I configure my network in functional clusters (lights, switches, sensors).
24 hours after config change this is visualization of my zigbee network, quite different from the first one and all nodes are connected (except for xiaomi buttons which report only ocassionaly so it is ok that they appear disconnected).
here are also all of my changes in configuration.yaml:
zha:
zigpy_config:
source_routing: true
ota:
ikea_provider: true # Auto update Trådfri devices
network:
channel: 11
ezsp_config:
CONFIG_MAX_END_DEVICE_CHILDREN: 16
CONFIG_SOURCE_ROUTE_TABLE_SIZE: 24
p.s. network channel is configured as this is the least populated frequency, so I guess the best one for zigbee
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Nice find and a good read for bedtime 
Actually I was looking for documentation on what kind of configurations are available to be set in configuration.xml? Seems not easy to find 
, just the basics are there. For example this source_routing was never mentioned in any documentation, also the last two configurations you have there that are to limit the routes and children.
I am on ZNP but I would assume config would be somewhat similar.
FYI still quite new to all this, moved from SmartThings which I out grew quickly. I have like 81 devices on my mesh, its okay, but I do lost connection to some of my switches from time to time, I have to hit them a few times for them to wake up. Not sure if this might just be a switch thing as it is an end device.