Sugarmate
Sign-up to Sugarmate (it's free).
Using the top left hamburger menu go to Settings.
Scroll down to Add Data Source, select Connect to Dexcom and copy the email address indicated.
STOP! Don't click I did it yet!
Open your master Dexcom app, go to Share (enable sharing if it isn't) and add the email from above as a follower.
Now click I did it in Sugarmate and wait until you also see you BG in the app.
Go back to settings and scroll down to External JSON. Enable it.
Note the http address here, this is the one you'll have to setup in the M5Stack device.
This is the string you will need to put instead of your Nightscout string:
sugarmate.io/api/v1/XXXXXX/latest.json
Remember that if you disable share, the Dexcom Share server goes down, or if you disable external JSON in Sugarmate, you will not have data on your M5Stack.