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Expected Behaviour:
-Raspberry Pi 3 Model B
-Spectrum Wifi router (SAX1V1R)
-All devices connected to wifi router use the wifi router ( 192.168.1.1) as DNS server
-With Pi then setup as the DNS server for the wifi router, all DNS queries for all devices connected to the wifi router (over wireless and direct ethernet to wifi router) should show in Query Log
-All devices connected to wifi router should show as active clients
Actual Behaviour:
The only active client showing in Pihole is 127.0.0.1 and only a few queries show in Query Log.
After a cursory read-through of the router "setup" information and reading other posts from Spectrum router users, it looks like you probably can only specify the DNS server the router itself uses for it's WAN connection and cannot specify what DHCP clients use. In that case, what you've configured would appear to be the only option available, unless something has changed.
Given the above, the only "client" your Pihole has is indeed the Spectrum router, so as far as Pihole is concerned the only client making DNS requests is the router (the clients of the router aren't visible directly to Pihole).
The issue with only a few queries showing may be due to the possibility you set more than one DNS server in your router's setup (primary and secondary DNS, for example). Someone with access to see your debug log can answer that for sure. For Pihole to get all the queries, it has to be the only DNS server specified.
If the Spectrum router let's you specify LAN DHCP options, that would be the place to specify Pihole's address for the DNS servers; that may not be an option for you, though.