A final update. I faffed around for hours trying to get TOR to run on the Pi Zero W, as an Onion router, to no avail. I can still use the Pi Zero W as an access point for the IP camera subnet, and it works well doing that, and doing this reduces the amount of wireless traffic on the main wireless router, which is running on a different subnet. I'll probably get around to writing up how I've used some cheap Chinese outdoor IP cameras, to connect to this subnet, and then to a Raspberry Pi 3 CCTV server, running MotioneyeOS, at some point. Suffice to say that it works well in this role, and significantly extends the outdoor WiFi range.
To resolve the desire to run TOR on a router, I discovered that the OpenWrt team had ported a version that runs on a very cheap and small, mini-router, the WT3020 series. I bought a WT3020F, which has both a LAN and WAN Ethernet port, plus WiFi and a USB storage port, for less than £15 delivered, from Banggood: https://www.banggood.com/NEXX-WT3020F-300Mbps-Portable-Mini-Wireless-WIFI-NAS-Router-AP-Reapeater-Support-USB-Flash-Drive-p-1108743.html
The mini-router arrived this morning (took around two weeks to get here) and I set about connecting to it (it has a firmware coded back door that you can Telnet into) and installing first OpenWrt, then installing OnionWRT.
After around 10 minutes or so I had a working TOR router, that I can connect to via a separate wireless subnet, and that routes all traffic via the TOR network. It's pretty secure, but is only as private as you make it, as unless used with care your identity can still be leaked. Having said that, TOR can be pretty private if used carefully, and it's certainly a heck of a lot more secure than normal web use. The WT3020 is believed to be free from any hard-coded backdoors, and a quick look that I did with a packet sniffer (Wireshark) shows there's no obvious unusual traffic, so I think it's probably safe enough. I'm sure that if there was an issue with these then some of those that have been hacking them would have spotted it by now.
I've written up some instructions as a text file (so the commands should be easy to cut and paste into a Telnet terminal), that illustrate, step by step what I did to get this working. Nothing I've done was my own work; all I've done is filter some of the out of date information on hacking these mini-routers and use some up to date links (there are a lot of dead links and out of date information around for these things, I found) .
WT3020F TOR mini router.txt