the IPv6 is set to configure 'automatically' i will change to 'link-local'. Note, my desktop version of airport utility looks different then your screen shot on this point. i am using version 6.3.6.
I still use Mountain Lion for my regular computer.. resort to El Capo when forced to.
"3rd Floor Hallway" is model 1408
"Service Window" which is not pictured but has the same drop off issue is model 1408
I have run these in various extend network configurations and found them working fine when initially installed and then drop off.
It is interesting the older Gen 1 behaves itself.
Let me suggest a different method of setup.
I think your "mesh" setup has inherent weakness.. in that units that wireless extend cannot be forced to connect to a particular AP. So things can get pretty tricky. It would work better with all connected by ethernet or all connected by wireless extend.. but the main culprit could be the AC model extreme. I think the DNS in this can be faulty.. and what is happening is the units fail because they are lost on the network..
And this is where more pro products would tell you what is happening.
Our ISP had us set up with windows devices and we were not happy with the performance.
I was suggesting something more like Ubiquiti.. pro type products but not over expensive.
Anyway let's pursue the problem.
What I want you to do is setup the two wired units, 1408 in static IP mode and to handle their own dhcp.
The setup is here.
Re: airport time capsule keeps disconnecting
I think bridge connection has a weakness.. especially if the DNS in the main router goes phfft.
So in your particularly layout.. set the main router DHCP server to 10.0.1.2 - 10.0.1.100 and reduce the DHCP lease time to 1 hour.
Now take your pick. Set say 3rd floor hallway to static IP, 10.0.1.101 and set its DHCP range to 10.0.1.102-150
Set Service Window to 10.0.1.151 and set its DHCP range to 10.0.1.152-200
Both of them with lease times also of 1hour.
For DNS setting in those two units.. you can use your main router.. 10.0.1.1 but then also add either your ISP or 8.8.8.8 google public DNS.
It might well be also worthwhile having got all that done.. to use dhcp reservation in the main router so you actually apply the IP to it as well.. I have to check again.. I have forgotten if Apple can apply dhcp reservation outside dhcp range or not. But what we are trying to do is ensure the wired units stick in the ARP table of the main router.. they should but I am not sure it isn't worth having two bites at the Apple (sic).
This also allows you to always be able to pick up the fixed IP units using Configure Other in the main menu of the airport utility.. This has also helped me when items dropped out of sight.
This might initially look pretty complicated.. it isn't. Actually it is dead simple.. You are not routing .. you are simply dividing and moving dhcp load from the main router to peripheral units. It is still one flat network.. anything is in theory able to connect to anything else.
But the stability of this I found gave much improved results.. YMMV .. !!
I can give you more specifics if you need them.. I could almost replicate what you have with the equipment I have here.. with some substitution of bits and pieces. Extreme for express.
I also would say when I was building these systems.. I found the 7.6.4 fell over more often than 7.6.1 ever did.. it was 100% reproducible.. if I downgraded the reliability improved.. upgraded it got worse. But I haven't done much since I put in the AC model (TC in my case) where the whole setup became a lot less reliable.. which leads me to think DNS issue there could be the hidden problem.
I will add a few screenshot..
