I finished my grind, with a total of 25 demiatma earned. According to the Occult Crescent achievements I did exactly 100 CEs and 175 Fates. Taking the stated rates of 20% for a CE and 5% for a Fate gives me an expected value of 28.75 Demiatma, so I was actually slightly unlucky.
As a one-time grind it was excellent, I think the devs tuned it perfectly. I'm very very glad to not have the lazy tomestone relics of Endwalker. I have plenty of complaints about the current direction of the game, but they did really well here.