disqus_a4OlbSVOoE Sorry! I read too fast and thought because there was just this thread that you were the original poster.
As is the case with banking apps, the issue may be not only whether the hardware is supported but also whether the hardware is running an approved OS. For many years Google's checks were easy to spoof, but that time is coming to an end. If Dexcom is doing what some banking apps do, which is using hardware-supported attestation of the OS and the app, then it likely won't be possible to evade that. The author of the BYOD app might know whether or not hardware attestation is the issue.
It seems that one UK bank, Starling, has decided to code support for GrapheneOS into their app. In a technical sense Dexcom probably could do the same. But it's possible there might be regulatory pressure for them to not.
Reading around a little online, it seems Dexcom has a standalone display "receiver" device which seems like it's thinner and lighter than a phone and has longer battery life. That's probably not a great solution, but it might be more workable than carrying around two phones.