ConfigManager VS AutoConnect

This can get you 80% of the way, you'll need to add some hooks for the button toggling, but its all doable https://github.com/nrwiersma/ConfigManager

And for Wifi management on ESP32, this awesome repo that "does it all": https://hieromon.github.io/AutoConnect/ -- this repo creates its own access point you can connect to with your phone to pop-up a "captive portal" (like you see at hotels or airports) to enter your own Wifi credentials -- this is only done once -- after that, the ESP32 will connect to the last known good Wifi network. If you don't want that, just use the built-in Wifi functions to create a local Wifi hotspot to connect to. Then creating web pages on the ESP32 is easy -- you can use the "PageBuilder" JSON syntax, or host regular HTML/JS/CSS files on the SPIFF/LittleFS partition and server those in response to incoming requests.

Or if you don't want to have a hardcoded ssid and password in your program, you can use the AutoConnect library

I want the device to be portable, so I want the esp32 to automatically reconnect to Wi-Fi every time I switch it off and back on. I have used this library, and it serves the exact purpose of what I want to do in my project : https://github.com/Hieromon/AutoConnect

I want to save power on a battery powered sensor but I also want to have AutoConnect set up to set up and configure the wifi. Since having the wi-fi running it can take up a lot of energy I would like to make sure that none of the battery is used during the programming set up. I was thinking that the AutoConnect would only run when it detects that it is plugged into a 5v usb port. I also want a switch so that the usb can power either the sensor (lower power) or the wifi configuration (high power) parts of the program. How would I best accomplish this?

It is this one right: https://github.com/Hieromon/AutoConnect ?