I appreciate what you're trying to do.
Unfortunately, it is very difficult to do a proper comp because of the days in release, as you point out.
However, games, in the launch month, experience an exponential decay rate. Sales start at their highest point, decline sharply for a number of days, before smoothing out.
Random internet picture as example (although not completely accurate, as some seasonality bumps occur on Saturdays and Sundays, with mid week having small dips from the curve, and of course the curve will not bottom out at 0. It's just illustrative):
Y would be sales, and X would be time.
Games do not sell X,000 copies on day 4 and the same X,000 copies on day 15. The earlier the day, the higher the % of the overall sales in the period that day accounts for.
But yes, monthly comparables are terrible. And yes, if you lined up all the games and looked at the sales after X number of days, then it would be a better comparison. But regardless, the gap here is so large as to make that kind of comparison meaningful, but not result changing.
Enjoy the game. The franchise isn't going anywhere, and some dumb sales number of a product shouldn't impact anyone's enjoyment of it.