Answer: Yes, dnsmasq is on my router.
But I have to ask my question a 3rd time... Does this mean I MUST use pi-hole as both DHCP and DNS server or can I just use it as DNS?
I will try method 2 from this document: How do I configure my devices to use pi-hole as their DNS server
I used method 1, and just changed the DNS server in TomatoUSB to the pi-hole server. Notice that in the screenshot on that page, "Use DNSMasq for DHCP" is CHECKED. This option appears to allow DNSMasq to continue to function as DHCP server, so that's what I was going for. I will try method 2, which tells dnsmasq to use pi-hole as the DNS server.
What I'm not clear on is that I then have to configure pi-hole to use my router as the upstream DNS server, but my router thinks that pi-hole is the DNS server.. This seems like a circular reference to me...