Install seems to have gone fine, and everything's working--what am I missing?

Nice

From earlier posts I'd thought it might be necessary to turn on passthrough and avoid double NATting for the Deco system to work satisfactorily, and make lots of other changes, but none of that turned out not to be necessary: I just moved the master Deco's WAN cable from the Spectrum box into one of the 1-gig ports on the AT&T BGW, and blammo, everything worked. 

You have a double NAT configuration with the BGW320 and your main Deco's WAN port connected to the BGW320 unless you specifically setup your Deco in AP mode.

Should it work fine for most customers like you, yes as long as you don't have a Game console where you want an Open NAT configuration.

You also should consider reducing the Power level on the BGW320's Wi-Fi to reduce interference with your Deco Mesh system. It is very easy to manage for both 2.4 GHz and 5 GHz (each have a Power level setting)

http://192.168.1.254/cgi-bin/wconfig.ha

Dave

Yup, I understand why I might want to avoid double NAT for some uses, and also that I might have wifi interference. The former isn't an issue for me, and I did a reasonably detailed wifi scan which told me the BGW's wifi wasn't going to be a problem (and I'll certainly depower it or turn it off if things degenerate). At this point the BGW is having trouble getting its signal very far in any case, and it and the Decos seem to be managing themselves well (without any impetus from me they've chosen reasonably separate channels, for example).

Most of us will shutoff the radios in the BGW to avoid unnecessary interference.  You will hopefully come to recognize the benefits of efficiency and configure your Deco into AP mode.  In practical terms your devices will pickup IP addresses in the 192.168.1.64-253 range.  

I don't especially want to make my ~50 IoT devices viewable to AT&T, and for my purposes (which, remember, don't require gaming-quality responsiveness, or outside address access) the speeds I'm getting even double-NATted are just fine. I don't see any particular advantage, under the circs, to replacing my familar 10.x.x.x address space.

I did experiment with shutting off the BGW radios, though. I ran into the first suite of problems, namely the gateway not actually shutting them off, and then telling me another client had locked access to the configuration screens. When I did manage to turn them off there was almost no discernible change on the Deco networks. I'm in a medium-dense environment (there are about 25 APs visible to my wifi scanner in addition to mine, so not as bad as an office or apartment building but still notable), and so the various wifis all chase each other around looking for unloaded channels, and the BGW doesn't seem to be making things any worse.

I don't see any particular advantage, under the circs, to replacing my familar 10.x.x.x address space.

There really is not an advantage based on your description of your IoT devices and you home Wi-Fi usage. The comment from @gr8sho about the 192.168.1.0 network is just the default for the Gateway's LAN. You are able to customize it to a 10.x.x.x network or even 172.x.x.x.

Dave

Well, no, AT&T doesn't allow 10.x.x.x on its router, just 196 or 172.

Since I'm a bit of an empiricist, I did the obvious experiments. BGW wifi radios on, Deco in router mode (so double NAT). Direct wifi connection to BGW wifi (and hence 192.169.1.x address) yields about 610/610, connection via equally close master Deco (that is, the one that's ethernetted to the BGW, so double NAT 10.x.x.x) yields maybe 10-20 less each way. Distant connection via wifi to nearby meshed Deco (but distant from master Deco) yields about 80-90 each way, sometimes up to 150 or so.

First, turned off BGW radios. No noticeable effect. 

Second, switched Deco to AP mode, waited for ~50 clients to get new addresses in 192.168.1.x and AT&T to complete security checks (yes, I turned on Active Armor, we'll delve into that another day). No noticeable effect.

So technically I'm sure that the double NAT and the radio interference have an effect--I'm reasonably adept network-wise, so of course that has to be true. But it's sort of like whether adding salt to water changes the cooking time for spaghetti: physics (or is it chemistry?) says it raises the boiling temperature so the cooking time must be less, but guess what, the difference isn't noticeable.

So long as my use cases don't change, my original comment stands: don't do any fancy configuration (other than naming wifis, of course), plug everything in, and blammo, it works.

Well, no, AT&T doesn't allow 10.x.x.x on its router, just 196 or 172.

Good pointI forgot AT&T decided to break the rules and steal the 10.x.x.x network in 2012.

Dave