What a nightmare!
Forgive my laymans input, but I have found that WPA2 is a non trivial cryptographic load for these 400mhz MIPS routers... even with only a few wifi devices I have seen 50%+ CPU load during use that disappeared when offloaded to another device. I do not think the wifi client limit is at all related to DHCP.
As a stop gap solution you could add any available wifi routers as dumb APs by disabling their DHCP servers, giving them a non conflicting IP on the same subnet and putting them on different SSIDs. Split 3 ways, channel 1,6,11 (12?), you could cover the houses 30 or so devices.
I'm sure more clever things could be done sharing the same SSID, but if I wanted something to work thats where I would start.