Personally Iam not too keen to use proprietary software like Nine on something robust like Grapheneos.
What Iam using for years now with a slight discomfort regarding security is the good old Mail and Exchange app from Lineageos:
They work very well with Grapheneos as well. Lineageos project stopped their development in 2020 due to the maintenance overhead compared to the amount of users who mostly chose some other client anyway.
Why does Grapheneos not pick up those two apps in order to add native exchange support ?
PS.: If you install those apps you need to grant the access rights manually. They are not designed to ask for it since they were originally shipped with the rom and not installed separately.