Did I do something stupid?
Obviously.
Look at what should happen if you navigate to http://192.168.1.232/server, assuming that the Arduino's address is 192.168.1.232:
server.on("/server", HTTP_GET, [](){
if (relay1State == 1) {
relay1StateString = "True"; }
else {
relay1StateString = "False"; }
});
That is ALL that happens. Nowhere do you do anything with relay1State or relay1StateString.
What ARE you expecting to see happen?