I'd suggesting changing calculations to the last release of content (DLC, EP) vs release of next iteraition.
Using "last release of content" for the previous game in the series gives us less to compare with, because the original Civilization game had no 'additional content' - at least not officially. Civilization II had two Expansion Packs, but both were a combination of 'official' content (from the development team) and Mod content, so really cannot be compared to later 'All Official' Packs.
Of the remaining games, here are the figures for the last Expansion Packs:
Civilization III: Conquests - released in November 2003, 25 months after the original game, 23 months before Civ IV came out
Civilization IV: Beyond the Sword - released in July 2007, 21 months after the original game, 38 months before Civ V was released
Civilization V: Brave New World - released in March 2013, 30 months after the original game, 42 months before Civ VI was released
Civilization VI: Gathering Storm - released February 2019, 28 months after the original game,
Civilization VI: New Frontier - released May 2020, 43 months after the original game
Note that New Frontier was billed as a 'Season Pass' rather than an Expansion Pack
Also note that there is remarkable consistency in the timing of the second Expansion Pack for all the games: 21 - 30 months, average 26 months with 2 out of 4 being within 2 months of the average, and the latest span at 28 months actually being shorter than the previous game's (30 months)
Finally, there is much less consistency on the time between the Expansion Pack and the next game: 23 to 42 months, average 34.3 months with individual times between 4 and 11 months different from the average. On the other hand, New Frontier Pass came out almost exactly when one might have expected the next Civ game to be released based on the previous intervals.
What any of this actually means is harder to determine. The game industry is by no means the same in 2022 as it was in 1991 (the original Civilization) or 2003, the earliest date in this set. Based on the times here, and especially the timing for NFP, I suspect that Civ VII development may have started as far back as 2019 but ran into problems even before the COVID distractions - that would be consistent with the development/release cycles of the previous games and expansion packs in the series.