Use this information at your own risk!
I am a mechanic ( a really old one) but I have been known to give programming a try . So I figured out how to add the ESP32 Thing Plus C to the Arduino SparkFun board list. I was not hard.
I work on a Raspberry Pi so this applies to Debian based systems. My Arduino is installed in /opt.
For whatever reason on my system there is 2 places that the files are.
/home/pi/.arduino15/packages/SparkFun/hardware/esp32/1.0.0/boards.txt and /home/pi/.arduino15/packages/SparkFun/hardware/esp32/1.0.1/boards.txt.
Using mousepad (a simple plain text editor) I copied the ESP32 Thing Plus board info to another document and used the search and replace to change names by adding a _c. I then copied that document back to the boards.txt file fust below the ESP32 Thing Plus info. Save and now you need to create an pins file. To the variants file I added a file folder esp32_thing_plus_c. In that folder I saved a modified copy of the on from the ESP32 Thing Plus. The changes are, change SDA from 23 to 21, SS from 33 to 5, MOSI from 18 to 23, SCK from 5 to 18, and A5 from 4 to 35. I figured out the changes from read the schematics.
I ran the SD card test, the Fuel gauge test and the I2C scanner power test with no problems.
Dale the mechanic (FAA A&P)