This is beta software, What may work on this release may not work on the next beta. Something like Dexcom I would not take the risk to install iOS15 on my main device.
OP, please take heed to what @Surfer13134 said. With your condition, you shouldn't take unnecessary chances using beta software, especially when your life is on the line.
Wait until iOS 15 has been released and you are positive that Dexcom has updated their app to support the new OS.