I have some experience with Nutrisense Continuous Glucose Monitors (CGM) , and the experience is very user-friendly through the app. The problem is it costs $225/mo, and most of that cost is in the software.
I think you can get Libre CGMs for $40/mo but they don't come with any software or insights.
There has to be an open source solution, right? On google I saw a few people built their own projects, but I'm wondering if anyone here is familiar with the range of open source in this area and what the best app is.
Sure there is: https://xdrip.readthedocs.io/en/latest/ This is for android, there other projects based on this code for iOS as well.